Move normalize_tags (which processes metatags) from before_save to
before_validation. This is so that it runs as early as possible, before
strip_source / parse_pixiv_ids / updater_can_change_rating, so these
callbacks can handle source/rating changes from metatags.
Fixes a couple bugs:
* Ratings locks were ignored when using rating:s metatag (regression in 0006b76)
* Pixiv ids weren't updated when using source:blah metatag.
Note: this means that `post.update_attribute(:tag_string => "art:bkub)`
is now wrong. This is because update_attribute runs callbacks but not
validations, so it doesn't process metatags from the tag string.
`update` or `update_attributes` must be used instead.
07921d2c88