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.
that's the latest commit made to deviantart files before switching from
the developer API to the Javascript backend from the new "Eclipse"
frontend.
This is necessary because it's basically impossible to download posts
now with the JS backend without being logged in, i.e. having the cookies
from a logged in user, which can't be used for very long even if
exporting them from a browser. You would have to save the cookies
deviantart sends you back via the "Set-Cookie" header in a database
somewhere in addition to the other added complexity.
also
* (temporarily) replace HttpartyCache with HTTParty as it's long been
removed
* fix one case of "last argument as keyword parameter"
* change repository url (5d1a1cc87e)
* remove self-explanatory comment
