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.
Remove the `image_url` method from source strategies. This method would
return only the first image if a source had multiple images. The
`image_urls` method should be used instead. Tests were the main place
that still used `image_url` instead of `image_urls`.
Also make post replacements return an error if replacing with a source
that contains multiple images, instead of just blindly replacing the
post with the first image in the source.
* Move the source normalization logic out of the post model
and into individual sources' strategies.
* Rewrite normalization tests to be handled into each source's test,
and expand them significantly. Previously we were only testing
a very small subset of domains and variants.
* Fix up normalization for several sites.
* Normalize fav.me urls into normal deviantart urls.
