Manually controlling user access in message

[edemaine]

As an alternative to @mentions, it would be nice to have an interface for adding users to accessibility list (with list of users / autocomplete), presumably under Actions. And deleting users. [Jeffrey] #274 might satisfy most of this.

Could also imagine separating read, edit, and admin (changing permissions) access. Related to #79.

For 6.042-style collaboration, would be nice to have definable "subgroups" that you can share with. Ideally subgroups could be listed in subgroups too, as in Moira.

[edemaine]

Most of this has been implemented in #503. The subgroup idea has been preserved in a new #514.