This notifies the user that the remote user doesn't exist, instead of
showing a blank page.

Ref T627

This changes all variables and functions from using Suspend{ed} to using
Silence{d} as well as documentation, errors and logging.

This puts the verbiage more in line with what the feature does, and
leaves room for other moderation controls in the future.

NOTE: this includes no backend refactoring, which may be confusing. We
should rename things to fit ASAP.

Ref T661

This renders all requests for that user's posts, collections and related
ActivityPub endpoints with 404 responses.

While suspended, users may not create or edit posts or collections.

User status is listed in the admin user page

Admin view of user details shows status and now has a button to activate
or suspend a user.

- Database schema changes, removing obsolete custom domain-related code
- Missing user structs
- Setup verbiage changes
- Missing routes
- Missing error messages

This includes config changes, collections, posts, some post rendering
funcs, and actual database connection when the server starts up.