Fix #5366: Baraag broken.

* Use fixed access tokens instead of fetching an access token with the OAuth flow. This assumes
  access tokens won't expire, which seems to be true for the default app-level access token, unless
  you manually regenerate it. Fixes the OAuth flow not working on Baraag for some reason.

* Eliminate the MastodonApiClient class. Just inline it in the extractor instead.

Downstream users will need to update their configs to set the `pawoo_access_token` and
`baraag_access_token` config options.

config/danbooru_default_config.rb

@@ -381,19 +381,11 @@ module Danbooru
     end
 
     # http://tinysubversions.com/notes/mastodon-bot/
-    def pawoo_client_id
+    def pawoo_access_token
       nil
     end
 
-    def pawoo_client_secret
-      nil
-    end
-
-    def baraag_client_id
-      nil
-    end
-
-    def baraag_client_secret
+    def baraag_access_token
       nil
     end