Follow-up to #2599. When a domain block with `reject_media` is
added or `rake mastodon:media:remove_remote` is invoked, mastodon
deletes the locally cached attachments and avatars but does not
reflect that change in the database, causing the `file` fields to
still have values. This change persists the deletion in the
database and sets the attachment type to unknown.

This also introduces a one-off rake task that sets all attachments
without a local file to the "unknown" type. The upgrade notes for
the next release should contain a post-upgrade step with
`rake mastodon:media:set_unknown`.

* Add coverage for domain block service with silence

* Get rid of warning about find_each and order

* Move domain_block to attr_reader

* Move optional clear_media into silence_accounts method

* Use blocked_domain method to reduce passed vars

* Extract blocked_domain_accounts method to find accounts on the domain

* Extract media_from_blocked_domain method to find relevant attachments

* Separate destruction of account images and account attachments

Create MediaAttachment but without actual file download when domain is blocked with reject_media set to true
Clean up old media files when creating a new domain block with reject_media set to true
Return remote_url in media attachments API if local file is not present
Undo domain block action in admin UI
Ability to enable reject_media from admin UI