appeals: raise appeal limits.

The old limit was one appeal per day. The new limit is based on your
upload limit. Each appeal costs 3 upload slots. If you have 15 upload
slots, then you can appeal up to 5 posts at once, but you won't be able
to appeal or upload more until your appeals are approved or rejected. If
you have unlimited uploads, then you have unlimited appeals.

app/models/user.rb

@@ -349,6 +349,11 @@ class User < ApplicationRecord
       end
     end
 
+    def is_appeal_limited?
+      return false if can_upload_free?
+      upload_limit.free_upload_slots < UploadLimit::APPEAL_COST
+    end
+
     def upload_limit
       @upload_limit ||= UploadLimit.new(self)
     end