Fetch metadata from Pixiv's smartphone API instead of scraping the HTML.

Also, normalize all URLs into old-style full-size URLs (e.g.
http://i2.pixiv.net/img78/img/demekyon/46187950.jpg).

Fetching the metadata from the API is necessary because for old-style
Pixiv URLs we need to know the image directory (e.g. "img04"), and that
information isn't available in the HTML.
Fix #2261: Normalize new-style Pixiv URLs into old-style URLs.
Prepopulate the source field with the normalized URL.

This allows the artist finder to use the old-style Pixiv URL
when a new-style URL is supplied using the upload bookmarklet.
Check for previously uploaded posts using the normalized source URL to catch more duplicates.
Handle revised images correctly.
Make VCR disallow unexpected HTTP requests.

This makes it so that test cases fail when a method makes HTTP requests
it isn't supposed to.
Add tests for downloading new-style Pixiv URLs.
Regenerate VCR cassettes for Pixiv download tests.
Don't rewrite old-style full-size image URLs.

This avoids making unnecessary HTTP requests for old-style full-size
image URLs that don't need to be normalized.

Also, fix the regex for small manga pages so it doesn't attempt to
rewrite new-style URLs.
Simplify the illust ID and gallery index regexes.
Merge branch 'issue-2261' of github.com:evazion/danbooru into issue-2261

Conflicts:
	app/logical/downloads/strategies/pixiv.rb
	app/logical/sources/strategies/pixiv.rb