PLT-7503: Create Message Export Scheduled Task and CLI Command (#7612)

* Created message export scheduled task

* Added CLI command to immediately kick off an export job

* Added email addresses for users joining and leaving the channel to the export

* Added support for both MySQL and PostgreSQL

* Fixing gofmt error

* Added a new ChannelMemberHistory store and associated tests

* Updating the ChannelMemberHistory channel as users create/join/leave channels

* Added user email to the message export object so it can be included in the actiance export xml

* Don't fail to log a leave event if a corresponding join event wasn't logged

* Adding copyright notices

* Adding message export settings to daily diagnostics report

* Added System Console integration for message export

* Cleaned up TODOs

* Made batch size configurable

* Added export from timestamp to CLI command

* Made ChannelMemberHistory table updates best effort

* Added a context-based timeout option to the message export CLI

* Minor PR updates/improvements

* Removed unnecessary fields from MessageExport object to reduce query overhead

* Removed JSON functions from the message export query in an effort to optimize performance

* Changed the way that channel member history queries and purges work to better account for edge cases

* Fixing a test I missed with the last refactor

* Added file copy functionality to file backend, improved config validation, added default config values

* Fixed file copy tests

* More concise use of the testing libraries

* Fixed context leak error

* Changed default export path to correctly place an 'export' directory under the 'data' directory

* Can't delete records from a read replica

* Fixed copy file tests

* Start job workers when license is applied, if configured to do so

* Suggestions from the PR

* Moar unit tests

* Fixed test imports

model/config.go

@@ -8,6 +8,7 @@ import (
 	"io"
 	"net/http"
 	"net/url"
+	"path/filepath"
 	"strings"
 	"time"
 )
@@ -1508,6 +1509,36 @@ func (s *PluginSettings) SetDefaults() {
 	}
 }
 
+type MessageExportSettings struct {
+	EnableExport        *bool
+	DailyRunTime        *string
+	ExportFromTimestamp *int64
+	FileLocation        *string
+	BatchSize           *int
+}
+
+func (s *MessageExportSettings) SetDefaults() {
+	if s.EnableExport == nil {
+		s.EnableExport = NewBool(false)
+	}
+
+	if s.FileLocation == nil {
+		s.FileLocation = NewString("export")
+	}
+
+	if s.DailyRunTime == nil {
+		s.DailyRunTime = NewString("01:00")
+	}
+
+	if s.ExportFromTimestamp == nil {
+		s.ExportFromTimestamp = NewInt64(0)
+	}
+
+	if s.BatchSize == nil {
+		s.BatchSize = NewInt(10000)
+	}
+}
+
 type ConfigFunc func() *Config
 
 type Config struct {
@@ -1538,6 +1569,7 @@ type Config struct {
 	WebrtcSettings        WebrtcSettings
 	ElasticsearchSettings ElasticsearchSettings
 	DataRetentionSettings DataRetentionSettings
+	MessageExportSettings MessageExportSettings
 	JobSettings           JobSettings
 	PluginSettings        PluginSettings
 }
@@ -1617,6 +1649,7 @@ func (o *Config) SetDefaults() {
 	o.LogSettings.SetDefaults()
 	o.JobSettings.SetDefaults()
 	o.WebrtcSettings.SetDefaults()
+	o.MessageExportSettings.SetDefaults()
 }
 
 func (o *Config) IsValid() *AppError {
@@ -1680,6 +1713,10 @@ func (o *Config) IsValid() *AppError {
 		return err
 	}
 
+	if err := o.MessageExportSettings.isValid(o.FileSettings); err != nil {
+		return err
+	}
+
 	return nil
 }
 
@@ -1998,6 +2035,35 @@ func (ls *LocalizationSettings) isValid() *AppError {
 	return nil
 }
 
+func (mes *MessageExportSettings) isValid(fs FileSettings) *AppError {
+	if mes.EnableExport == nil {
+		return NewAppError("Config.IsValid", "model.config.is_valid.message_export.enable.app_error", nil, "", http.StatusBadRequest)
+	}
+	if *mes.EnableExport {
+		if mes.ExportFromTimestamp == nil || *mes.ExportFromTimestamp < 0 || *mes.ExportFromTimestamp > time.Now().Unix() {
+			return NewAppError("Config.IsValid", "model.config.is_valid.message_export.export_from.app_error", nil, "", http.StatusBadRequest)
+		} else if mes.DailyRunTime == nil {
+			return NewAppError("Config.IsValid", "model.config.is_valid.message_export.daily_runtime.app_error", nil, "", http.StatusBadRequest)
+		} else if _, err := time.Parse("15:04", *mes.DailyRunTime); err != nil {
+			return NewAppError("Config.IsValid", "model.config.is_valid.message_export.daily_runtime.app_error", nil, err.Error(), http.StatusBadRequest)
+		} else if mes.FileLocation == nil {
+			return NewAppError("Config.IsValid", "model.config.is_valid.message_export.file_location.app_error", nil, "", http.StatusBadRequest)
+		} else if mes.BatchSize == nil || *mes.BatchSize < 0 {
+			return NewAppError("Config.IsValid", "model.config.is_valid.message_export.batch_size.app_error", nil, "", http.StatusBadRequest)
+		} else if *fs.DriverName != IMAGE_DRIVER_LOCAL {
+			if absFileDir, err := filepath.Abs(fs.Directory); err != nil {
+				return NewAppError("Config.IsValid", "model.config.is_valid.message_export.file_location.relative", nil, err.Error(), http.StatusBadRequest)
+			} else if absMessageExportDir, err := filepath.Abs(*mes.FileLocation); err != nil {
+				return NewAppError("Config.IsValid", "model.config.is_valid.message_export.file_location.relative", nil, err.Error(), http.StatusBadRequest)
+			} else if !strings.HasPrefix(absMessageExportDir, absFileDir) {
+				// configured export directory must be relative to data directory
+				return NewAppError("Config.IsValid", "model.config.is_valid.message_export.file_location.relative", nil, "", http.StatusBadRequest)
+			}
+		}
+	}
+	return nil
+}
+
 func (o *Config) GetSanitizeOptions() map[string]bool {
 	options := map[string]bool{}
 	options["fullname"] = o.PrivacySettings.ShowFullName