Get rid of `normalized_for_artist_finder?` and `normalizable_for_artist_finder?`.
This was legacy bullshit that was originally designed to avoid API calls
when saving artist entries containing old Pixiv direct image urls that
had already been normalized, or that couldn't be normalized because they
were bad id.
Nowadays we store profile urls in artist entries instead of direct image
urls, so we don't normally need to do any API calls to normalize the
profile url. Strategies should take care to avoid triggering API calls
inside `profile_url` when possible.
* 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.
