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/timerlayer/timerlayer.go

@@ -9112,6 +9112,22 @@ func (s *TimerLayerThreadStore) DeleteMembershipForUser(userId string, postID st
 	return err
 }
 
+func (s *TimerLayerThreadStore) DeleteMembershipsForChannel(userID string, channelID string) error {
+	start := time.Now()
+
+	err := s.ThreadStore.DeleteMembershipsForChannel(userID, channelID)
+
+	elapsed := float64(time.Since(start)) / float64(time.Second)
+	if s.Root.Metrics != nil {
+		success := "false"
+		if err == nil {
+			success = "true"
+		}
+		s.Root.Metrics.ObserveStoreMethodDuration("ThreadStore.DeleteMembershipsForChannel", success, elapsed)
+	}
+	return err
+}
+
 func (s *TimerLayerThreadStore) DeleteOrphanedRows(limit int) (int64, error) {
 	start := time.Now()