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

einterfaces/jobs/message_export.go

@@ -0,0 +1,13 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package jobs
+
+import (
+	"github.com/mattermost/mattermost-server/model"
+)
+
+type MessageExportJobInterface interface {
+	MakeWorker() model.Worker
+	MakeScheduler() model.Scheduler
+}

einterfaces/message_export.go

@@ -0,0 +1,14 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package einterfaces
+
+import (
+	"context"
+
+	"github.com/mattermost/mattermost-server/model"
+)
+
+type MessageExportInterface interface {
+	StartSynchronizeJob(ctx context.Context, exportFromTimestamp int64) (*model.Job, *model.AppError)
+}