Fix PostgreSQL load when linking in announcements (#13250)

* Fix PostgreSQL load when linking in announcements

Fixes #13245 by caching status lookups

Since statuses are supposed to be known already and we only
need their URLs and a few other things, caching them should
be fine.

Since it's only used by announcements so far, there won't
be much statuses to cache.

* Perform status lookup when saving announcements, not when rendering them

* Change EntityCache#status to fetch URLs instead of looking into the database

* Move announcement link lookup to publishing worker

* Address issues pointed out during review

app/lib/entity_cache.rb

@@ -7,6 +7,10 @@ class EntityCache
 
   MAX_EXPIRATION = 7.days.freeze
 
+  def status(url)
+    Rails.cache.fetch(to_key(:status, url), expires_in: MAX_EXPIRATION) { FetchRemoteStatusService.new.call(url) }
+  end
+
   def mention(username, domain)
     Rails.cache.fetch(to_key(:mention, username, domain), expires_in: MAX_EXPIRATION) { Account.select(:id, :username, :domain, :url).find_remote(username, domain) }
   end