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.
Fix call to PixivUgoiraConverter.delay.

Should be `PixivUgoiraConverter.new.delay`, not PixivUgoiraConverter.delay`.
Merge pull request #2277 from evazion/ugoira-fix-model

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