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.

instead of updating it later. This is more correct behavior, prevents an
additional query, and ensures that potential collection-creation when
claiming posts also respects the default (which it didn't before).

Ref T675

This adds a new `default_visibility` config value that lets an instance
admin set the visibility of newly created collections.

Ref T675

This is the crucial part where the hostName is needed for federation and
API clients. This change at least lets us know when we mess up like this
so the issue is easier to catch in the future.

This moves `hostName` to the `Collection` struct, where it's needed. The
field is populated after successful `GetCollection...()` calls.

This isn't the cleanest way to do things, but it accomplishes the goal.
Eventually, we should accept the AppCfg to `GetCollection...()` calls,
or make them `App` methods, instead of `datastore` methods.

Ref T613

We load it always, no need for a user config UI

This is a first stab at having a configurable code highlighting option,
similar to the MathJax rendering option. This change makes a checkbox
in the settings for code highlighting using the highlightjs.org
library.
What works: code highlighting in multi-user env is like I would
expect. single and anon(?) needs work

Things to resolve/consider:
- does the .IsCode test for code highlighting need to stay? At least
this and that should use the same version of the highlight.js lib.
- can the common templating part be 'included' somehow?
- the anon vs single-user vs multi-user code is not completely
clear (to me)
- bring js to local instead of cloudfare cdn (perhaps combine with
MathJax)

This helps with post importing and clients that want to support post
scheduling. It also changes how Collection.ForPublic() works, no longer
resetting the ID.

Closes T532

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