* Add "general" rating.
* Rename "safe" rating to "sensitive".
* Change safe mode to include both rating:s and rating:g.
* Treat rating:safe as a synonym for rating:sensitive.
* Link "howto:rate" in the post edit form.
Refactor ratings to not be hardcoded in various places. Make it so
all ratings are defined in Post::RATINGS.
Also make it so that you can search multiple ratings at once with `rating:q,e`.
Refactor source strategies to remove the `canonical_url` method.
`canonical_url` returned the URL that should be used as the source of
the post after upload. Now we simply use `Source::URL#page_url` to
determine the source after upload. If the source is an image URL that is
convertible to a page URL, then the image URL is used as the source. If
the source is an image URL that is not convertible to a page URL, then
the page URL is used as the source.
This simplifies source strategies so that all they have to care about is
implementing the `Source::URL#page_url` and `Sources::Strategies#page_url`
methods, and the preferred source will be chosen for posts automatically.
Fixes an issue where if you were uploading a multi-image source, and you
clicked on a thumbnail that was still processing, then the page wouldn't
refresh when the processing was complete.
