Change how artist URLs are normalized in artist entries. Don't try to secretly
convert image URLs to profile URLs in artist entries. For example, if someone puts a
Pixiv image URL in an artist entry, don't secretly try to fetch the source and
convert it into a profile URL in the `normalized_url` field.
We did this because years ago, it was standard practice to put image URLs in artist
entries. Pixiv image URLs used to contain the artist's username, so we used to put
image URLs in artist entries for artist finding purposes. But Pixiv changed it so
that image URLs no longer contained the username, so we dealt with it by adding a
`normalized_url` column to artist_urls and secretly converting image URLs to profile
URLs in this field. But this is no longer necessary because now we don't normally put
image URLs in artist entries in the first place.
Now the `profile_url` method in `Source::URL` is used to normalize URLs in artist
entries. This lets us parse various profile URL formats and normalize them into a
single canonical form.
This also removes the `normalize_for_artist_finder` method from source strategies.
Instead the `profile_url` method is used for artist finding purposes. So the profile
URL returned by the source strategy needs to be the same as the URL in the artist
entry in order for artist finding to work.
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