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.

This makes it so we can keep all schema changes in the `migrations`
module, instead of adding them there *and* in the schema files. It
fixes #92 and should prevent these kinds of issues in the future.

Now admins can choose a title for their About and Privacy pages; now
editable through the instance page editor.

This adds `title` and `content_type` fields to the `appcontent` table,
requiring a migration by running `writefreely --migrate`

The content_type field specifies that items we're currently storing in
this table are all "page"s; queries for fetching these have been updated
to filter for this type. In the future, this field will be used to
indicate when an item is a stylesheet (ref T563) or other supported
type.

Ref T566

This includes the appmigrations table in the schema files, and inserts
the current database version when running writefreely --init-db

This includes:

- A new `user_invites` config value that determines who can generate
  invite links
- A new page for generating invite links, with new user navigation link
- A new /invite/ path that allows anyone to sign up via unique invite
  link, even if registrations are closed
- Tracking who (of registered users) has been invited by whom

It requires an updated database with `writefreely --migrate` in order to
work.

This closes T556

This runs database migrations and obviates the need for manually running
changes.

Ref T509