Fix performance on instances list in admin UI (#15282)

- Reduce duplicate queries
- Remove n+1 queries
- Add accounts count to detailed view
- Add separate action log entry for updating existing domain blocks

app/models/unavailable_domain.rb

@@ -12,6 +12,8 @@
 class UnavailableDomain < ApplicationRecord
   include DomainNormalizable
 
+  validates :domain, presence: true, uniqueness: true
+
   after_commit :reset_cache!
 
   private