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.

app/models/artist.rb

@@ -22,6 +22,7 @@ class Artist < ActiveRecord::Base
 
     module ClassMethods
       def find_all_by_url(url)
+        url = Sources::Site.new(url).normalize_for_artist_finder!
         url = ArtistUrl.normalize(url)
         artists = []