MM-45009: Delete ThreadMemberships from "left" channels (#22559)

* MM-50550: Filter out threads from "left" channels v2

Currently leaving a channel doesn't affect the thread memberships of
that user/channel combination.
This PR aims to filter out all threads from those channels for the user.

Adds a DeleteAt column in the ThreadMemberships table, and filter out
all thread memberships that are "deleted".
Each time a user leaves a channel all thread memberships are going to be
marked as deleted, and when a user joins a channel again all those
existing thread memberships will be re-instantiated.

Adds a migration to mark all existing thread memberships as deleted
depending on whether there exists a channel membership for that
channel/user.

* Added migration files into list

* Fixes tests

* Fixes case where DeleteAt would be null

* Guard thread API endpoints with appropriate perms

* Deletes ThreadMembership rows upon leaving channel

* Minor style changes

* Use NoTranslation error

* Refactors tests

* Adds API tests to assert permissions on Team

* Adds tests, and fixes migrations

* Fixes test description

* Fix test

* Removes check on DM/GMs

* Change the MySQL query in the migration

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/channels/store/retrylayer/retrylayer.go

@@ -11563,6 +11563,27 @@ func (s *RetryLayerThreadStore) DeleteMembershipForUser(userId string, postID st
 
 }
 
+func (s *RetryLayerThreadStore) DeleteMembershipsForChannel(userID string, channelID string) error {
+
+	tries := 0
+	for {
+		err := s.ThreadStore.DeleteMembershipsForChannel(userID, channelID)
+		if err == nil {
+			return nil
+		}
+		if !isRepeatableError(err) {
+			return err
+		}
+		tries++
+		if tries >= 3 {
+			err = errors.Wrap(err, "giving up after 3 consecutive repeatable transaction failures")
+			return err
+		}
+		timepkg.Sleep(100 * timepkg.Millisecond)
+	}
+
+}
+
 func (s *RetryLayerThreadStore) DeleteOrphanedRows(limit int) (int64, error) {
 
 	tries := 0