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.

You might have to do `DELETE FROM pixiv_ugoira_frame_data WHERE data IS NULL;` before running `rake db:migrate` to clear out any invalid records.