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/app/channel.go

@@ -2518,6 +2518,9 @@ func (a *App) removeUserFromChannel(c request.CTX, userIDToRemove string, remove
 	if err := a.Srv().Store().ChannelMemberHistory().LogLeaveEvent(userIDToRemove, channel.Id, model.GetMillis()); err != nil {
 		return model.NewAppError("removeUserFromChannel", "app.channel_member_history.log_leave_event.internal_error", nil, "", http.StatusInternalServerError).Wrap(err)
 	}
+	if err := a.Srv().Store().Thread().DeleteMembershipsForChannel(userIDToRemove, channel.Id); err != nil {
+		return model.NewAppError("removeUserFromChannel", model.NoTranslation, nil, "failed to delete threadmemberships upon leaving channel", http.StatusInternalServerError).Wrap(err)
+	}
 
 	if isGuest {
 		currentMembers, err := a.GetChannelMembersForUser(c, channel.TeamId, userIDToRemove)