Make file attachment on MediaAttachment optional (#1865)

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

app/models/domain_block.rb

@@ -3,6 +3,8 @@
 class DomainBlock < ApplicationRecord
   enum severity: [:silence, :suspend]
 
+  attr_accessor :retroactive
+
   validates :domain, presence: true, uniqueness: true
 
   def self.blocked?(domain)