* The "Newest" tab of the forum is updated with new filter options.
* Which forums: All, Official, Community, My List
* Show: By threads, All posts
* The option for "Which forums" is saved in the user's preferences and set as
their default on future visits, similar to the Site Gallery "Whose photos"
option.
* So users can subscribe to their favorite forums and always get their latest
posts easily while filtering out the rest.
* Forum Moderators
* Add the ability to add and remove moderators for your forum.
* Users are notified when they are added as a moderator.
* Moderators can opt themselves out by unfollowing the forum.
* ForumMembership: add unique constraint on user_id,forum_id.
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