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/job.go

@@ -12,6 +12,7 @@ import (
 
 const (
 	JOB_TYPE_DATA_RETENTION                 = "data_retention"
+	JOB_TYPE_MESSAGE_EXPORT                 = "message_export"
 	JOB_TYPE_ELASTICSEARCH_POST_INDEXING    = "elasticsearch_post_indexing"
 	JOB_TYPE_ELASTICSEARCH_POST_AGGREGATION = "elasticsearch_post_aggregation"
 	JOB_TYPE_LDAP_SYNC                      = "ldap_sync"
@@ -50,6 +51,7 @@ func (j *Job) IsValid() *AppError {
 	case JOB_TYPE_ELASTICSEARCH_POST_INDEXING:
 	case JOB_TYPE_ELASTICSEARCH_POST_AGGREGATION:
 	case JOB_TYPE_LDAP_SYNC:
+	case JOB_TYPE_MESSAGE_EXPORT:
 	default:
 		return NewAppError("Job.IsValid", "model.job.is_valid.type.app_error", nil, "id="+j.Id, http.StatusBadRequest)
 	}