Set `default_content_type` default from settings.yml

Glitch-soc used to allow setting the default content type for new users by changing the setting in settings.yml.

Since the refactor of user settings, changing it had no effect.

Signed-off-by: Plastikmensch <plastikmensch@users.noreply.github.com>

app/models/user_settings.rb

@@ -16,7 +16,7 @@ class UserSettings
   setting :default_language, default: nil
   setting :default_sensitive, default: false
   setting :default_privacy, default: nil, in: %w(public unlisted private)
-  setting :default_content_type, default: 'text/plain'
+  setting :default_content_type, default: -> { ::Setting.default_content_type }
   setting :hide_followers_count, default: false
 
   namespace :web do