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

store/sqlstore/compliance_store.go

@@ -211,3 +211,36 @@ func (s SqlComplianceStore) ComplianceExport(job *model.Compliance) store.StoreC
 		}
 	})
 }
+
+func (s SqlComplianceStore) MessageExport(after int64, limit int) store.StoreChannel {
+	return store.Do(func(result *store.StoreResult) {
+		props := map[string]interface{}{"StartTime": after, "Limit": limit}
+		query :=
+			`SELECT
+				Posts.Id AS PostId,
+				Posts.CreateAt AS PostCreateAt,
+				Posts.Message AS PostMessage,
+				Posts.Type AS PostType,
+				Posts.FileIds AS PostFileIds,
+				Channels.Id AS ChannelId,
+				Channels.DisplayName AS ChannelDisplayName,
+				Users.Id AS UserId,
+				Users.Email AS UserEmail
+			FROM
+				Posts
+				LEFT OUTER JOIN Channels ON Posts.ChannelId = Channels.Id
+				LEFT OUTER JOIN Users ON Posts.UserId = Users.Id
+			WHERE
+				Posts.CreateAt > :StartTime AND
+				Posts.Type = ''
+			ORDER BY PostCreateAt
+			LIMIT :Limit`
+
+		var cposts []*model.MessageExport
+		if _, err := s.GetReplica().Select(&cposts, query, props); err != nil {
+			result.Err = model.NewAppError("SqlComplianceStore.MessageExport", "store.sql_compliance.message_export.app_error", nil, err.Error(), http.StatusInternalServerError)
+		} else {
+			result.Data = cposts
+		}
+	})
+}