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/models/status.rb

@@ -379,7 +379,7 @@ class Status < ApplicationRecord
           if TagManager.instance.local_url?(url)
             ActivityPub::TagManager.instance.uri_to_resource(url, Status)
           else
-            Status.find_by(uri: url) || Status.find_by(url: url)
+            EntityCache.instance.status(url)
           end
         end
         status&.distributable? ? status : nil