Add methods to Source::URL for determining whether a URL is an image
URL, a page URL, or a profile URL.
Also add more source URL tests and fix various URL parsing bugs.
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.
Add upload support for Pixiv Sketch. Fetch tags, commentary, and artist,
and rewrite sample images to full images.
Authentication isn't required. R18 images are hidden in the browser but
visible in the API.
