Fix upload tests to use VCR.
Add artist finding tests for new Pixiv URLs.

* Add tests for finding artists using the new Pixiv URLs in Artist#find_all_by_url.
* Add tests for the artist finder JSON API in ArtistsController#finder.
* Add tests for the artist page search form in ArtistsController#index.
Refactor normalize_for_artist_finder!

Refactors things such that Sources::Site has a normalize_for_artist_finder!
method that delegates to the strategy for the appropriate site. This way
any site that needs to normalize URLs for the artist finder can do so.
Move normalize_for_artist_finder! into model.

Move the call to normalize_for_artist_finder! from the artist
controller into Artist#find_all_by_url. This makes testing easier.
It also makes it so that URLs are normalized when using the search
form on the artist listing page.
normalize_for_artist_finder!: Don't crash on bad URLs

If we can't normalize the URL (because of bad IDs, it's malformed, or
the HTML page changed), just return the unnormalized URL.