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
