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