Move use_feedback_instance configuration into adhocracy.config
Short-circuit feedback.is_configured

If use_feedback_instance is not set, we should not query the database.
Add a configuration to not check for the feedback instance on every request

Currently, we're sending a database query for every single rendering of adhocracy, just for the feedback button, and just to verify that the instance exists.
Add a configuration option adhocracy.feedback_check_instance that can be set to false in order to not check every single time.
See hhucn/adhocracy.hhu_theme#401 for context.
Add an option to use feedback without categories

If set, all feedback entries will be without a category.
In my tests, setting these two feedback options roughly halves loading time of the front page (for anonymous users).
See also hhucn/adhocracy.hhu_theme#401 .