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/store.go

@@ -63,6 +63,7 @@ type Store interface {
 	Reaction() ReactionStore
 	Job() JobStore
 	UserAccessToken() UserAccessTokenStore
+	ChannelMemberHistory() ChannelMemberHistoryStore
 	Plugin() PluginStore
 	MarkSystemRanUnitTests()
 	Close()
@@ -160,6 +161,13 @@ type ChannelStore interface {
 	GetChannelUnread(channelId, userId string) StoreChannel
 }
 
+type ChannelMemberHistoryStore interface {
+	LogJoinEvent(userId string, channelId string, joinTime int64) StoreChannel
+	LogLeaveEvent(userId string, channelId string, leaveTime int64) StoreChannel
+	GetUsersInChannelDuring(startTime int64, endTime int64, channelId string) StoreChannel
+	PurgeHistoryBefore(time int64, channelId string) StoreChannel
+}
+
 type PostStore interface {
 	Save(post *model.Post) StoreChannel
 	Update(newPost *model.Post, oldPost *model.Post) StoreChannel
@@ -276,6 +284,7 @@ type ComplianceStore interface {
 	Get(id string) StoreChannel
 	GetAll(offset, limit int) StoreChannel
 	ComplianceExport(compliance *model.Compliance) StoreChannel
+	MessageExport(after int64, limit int) StoreChannel
 }
 
 type OAuthStore interface {