MM-9728: Online migration for advanced permissions phase 2 (#8744)

* MM-9728: Online migration for advanced permissions phase 2

* Add unit tests for new store functions.

* Move migration specific code to own file.

* Add migration state function test.

* Style fixes.

* Add i18n strings.

* Fix mocks.

* Add TestMain to migrations package tests.

* Fix typo.

* Fix review comments.

* Fix up the "Check if migration is done" check to actually work.

store/store.go

@@ -105,6 +105,7 @@ type TeamStore interface {
 	RemoveAllMembersByUser(userId string) StoreChannel
 	UpdateLastTeamIconUpdate(teamId string, curTime int64) StoreChannel
 	GetTeamsByScheme(schemeId string, offset int, limit int) StoreChannel
+	MigrateTeamMembers(fromTeamId string, fromUserId string) StoreChannel
 }
 
 type ChannelStore interface {
@@ -163,6 +164,7 @@ type ChannelStore interface {
 	GetChannelUnread(channelId, userId string) StoreChannel
 	ClearCaches()
 	GetChannelsByScheme(schemeId string, offset int, limit int) StoreChannel
+	MigrateChannelMembers(fromChannelId string, fromUserId string) StoreChannel
 }
 
 type ChannelMemberHistoryStore interface {