Media attachments are part of the association cache of statuses,
since they are presumed to be immutable. Unless this cache is
cleared manually, the statuses will continue to look like they
have media embedded.

* Admin interface for listing, adding and removing custom emojis

* Only display local ones in the list

* Allow domain blocks to reject media without silencing or suspending

* Fix typo

* Hide 'Reject media' button when superfluous, instead of disabling it

* Properly hide 'reject media' checkbox on page load if needed

This may happen when resubmitting the domain block form after an error.

* Don't ask whether undoing a media-only block should be retroactive

* Rename :media_only block to :noop

* Display :noop block as None in frontend, allow blocks that do nothing

* Remove 'coding' line auto-added by emacs

* Set domain_block by before_action

* Cast value with ActiveRecord::Type

* Batch update

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