There are two kinds of thumbnails that need to be rewritten. First case:
new /img-master/ URLs need to be rewritten to /img-original/ URLs like this:
http://i2.pixiv.net/c/600x600/img-master/img/2014/10/04/03/59/52/46337015_p0_master1200.jpg
=> http://i2.pixiv.net/img-original/img/2014/10/04/03/59/52/46337015_p0.png
This is what `rewrite_new_medium_images` does. In order to do this, it
has to use the Pixiv API to get the correct file extension.
Second case: Old small/medium size URLs need to be rewritten to full
size URLs like this:
http://i2.pixiv.net/img18/img/evazion/14901720_m.png
=> http://i2.pixiv.net/img18/img/evazion/14901720.png
But when the medium size URL is actually for a manga image, it needs to be
rewritten to the big manga URL instead:
http://i2.pixiv.net/img04/img/syounen_no_uta/46170939_m.jpg
=> http://i2.pixiv.net/img04/img/syounen_no_uta/46170939_big_p0.jpg
But we can't tell whether it's a manga image from the URL, so we have to
use the manga page count from either the HTML page or the API to
determine whether it's part of a manga gallery.
So in order to make this work, `rewrite_old_small_and_medium_images`
takes an `is_manga` flag. `Sources::Strategies::Pixiv#get` gets the
page count from the HTML and passes the `is_manga` flag on down through
the call chain until `rewrite_old_small_and_medium_images` gets it.
When `rewrite_old_small_and_medium_images` is called from
`Downloads::Strategies::Pixiv#rewrite_thumbnails`, the `is_manga` flag
isn't passed in because we didn't scrape the HTML. This causes
`rewrite_old_small_and_medium_images` to look it up in the API instead.
