search: drop special case for pixiv urls in source: metatag.

* Drop support for `source:pixiv/artist-name` searches. This was a hack
  that only worked on old pixiv urls that haven't been used for years.
* Replace the old SourcePattern(lower(source)) index with a trigram index.

test/unit/post_test.rb

@@ -2206,8 +2206,6 @@ class PostTest < ActiveSupport::TestCase
       assert_tag_match([],     "source:*.pixiv.net/img*/artist-fake/*")
       assert_tag_match([post], "source:http://*.pixiv.net/img*/img/artist-name/*")
       assert_tag_match([],     "source:http://*.pixiv.net/img*/img/artist-fake/*")
-      assert_tag_match([post], "source:pixiv/artist-name/*")
-      assert_tag_match([],     "source:pixiv/artist-fake/*")
     end
 
     should "return posts for a pixiv id search (type 1)" do