Fixes #12795

It was already possible to create domain blocks for TLDs, but those
weren't enforced, nor editable. This commit changes it so that they
are enforced and editable.

* Change domain blocks to automatically support subdomains

If a more authoritative domain is blocked (example.com), then the
same block will be applied to a subdomain (foo.example.com)

* Match subdomains of existing accounts when blocking/unblocking domains

* Improve code style

* When trying to block an already-blocked domain, provide a link to the block

* Fix styling for links in flash messages

* Allow blocks to be upgraded but not downgraded