nicoseiga: fix inability to login to nicoseiga.

NicoSeiga changed it so that on every login, you must enter a 2FA code
sent by email. This broke the NicoSeiga strategy. The fix is to just use
a static session cookie instead (and hope it doesn't expire, and isn't
tied to an IP).

The `nico_seiga_login` and `nico_seiga_password` config settings have
been removed from config/danbooru_default_config.rb and replaced by
`nico_seiga_user_session`. If you run your own Danbooru instance, you
will have to update your config file manually.

config/danbooru_default_config.rb

@@ -265,11 +265,9 @@ module Danbooru
       nil
     end
 
-    def nico_seiga_login
-      nil
-    end
-
-    def nico_seiga_password
+    # Your NicoSeiga "user_session" cookie. Login to NicoSeiga then use the
+    # devtools to find the "user_session" cookie.
+    def nico_seiga_user_session
       nil
     end