artists: clean up artist finding logic.

Rename Artist#find_all_by_url to url_matches and drop previous
url_matches method, along with find_artists and search_for_profile.

Previously find_artists tried to lookup the url, referer url, and profile
url in turn until an artist match was found. This was wasteful, because
the source strategy already knows which url to lookup (usually the profile
url). If that url doesn't find a match, then the artist doesn't exist.

app/logical/sources/strategies/base.rb

@@ -129,6 +129,8 @@ module Sources
         normalize_for_artist_finder.present?
       end
 
+      # The url to use for artist finding purposes. This will be stored in the
+      # artist entry. Normally this will be the profile url.
       def normalize_for_artist_finder
         profile_url || url
       end
@@ -139,8 +141,7 @@ module Sources
       end
 
       def artists
-        url = profile_url.presence || image_url.presence
-        Artist.find_artists(url)
+        Artist.url_matches(normalize_for_artist_finder)
       end
 
       def file_url

app/logical/sources/strategies/twitter.rb

@@ -66,14 +66,6 @@ module Sources::Strategies
       ""
     end
 
-    def artists
-      if profile_url.present?
-        Artist.find_artists(profile_url)
-      else
-        []
-      end
-    end
-
     def artist_name
       return "" if api_response.blank?
       api_response.attrs[:user][:screen_name]