Fix count numbers from ActivityPub not being saved (#4899)

They are marked as read-only by Rails, but we know what we are doing,
so we are un-marking them as such.

The mastodon:maintenance:update_counter_caches task is not really
supposed to be run anymore (it was a one-time thing during an upgrade)
however, just in case, I have modified it to not touch ActivityPub
accounts.

Also, no point writing to logger from these rake tasks, since they
are not to be run from cron. Better to give stdout feedback.

app/models/account.rb

@@ -172,6 +172,10 @@ class Account < ApplicationRecord
   end
 
   class << self
+    def readonly_attributes
+      super - %w(statuses_count following_count followers_count)
+    end
+
     def domains
       reorder(nil).pluck('distinct accounts.domain')
     end