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/store.go

@@ -344,6 +344,7 @@ type ThreadStore interface {
 	PermanentDeleteBatchThreadMembershipsForRetentionPolicies(now, globalPolicyEndTime, limit int64, cursor model.RetentionPolicyCursor) (int64, model.RetentionPolicyCursor, error)
 	DeleteOrphanedRows(limit int) (deleted int64, err error)
 	GetThreadUnreadReplyCount(threadMembership *model.ThreadMembership) (int64, error)
+	DeleteMembershipsForChannel(userID, channelID string) error
 
 	// Insights - threads
 	GetTopThreadsForTeamSince(teamID string, userID string, since int64, offset int, limit int) (*model.TopThreadList, error)