Fix frame data records being created for non-ugoira posts.

* Prevent non-ugoira posts from getting dummy frame data records with
  all the columns set to null.
* Make `data` and `content_type` columns NOT NULL, since they should
  always be present.
* Remove the timestamps while we're at it, since they don't seem
  necessary for anything.

db/structure.sql

@@ -2367,10 +2367,8 @@ ALTER SEQUENCE notes_id_seq OWNED BY notes.id;
 CREATE TABLE pixiv_ugoira_frame_data (
     id integer NOT NULL,
     post_id integer,
-    data text,
-    content_type character varying(255),
-    created_at timestamp without time zone,
-    updated_at timestamp without time zone
+    data text NOT NULL,
+    content_type character varying(255) NOT NULL
 );
 
 
@@ -7119,3 +7117,5 @@ INSERT INTO schema_migrations (version) VALUES ('20140725003232');
 
 INSERT INTO schema_migrations (version) VALUES ('20141009231234');
 
+INSERT INTO schema_migrations (version) VALUES ('20141017231608');
+