We need a way to disable certain checks programatically, e.g. on Discourse hosting. This PR adds a configuration option for this, and makes it so that disabled checks aren't run as part of #run_all.
We support a low-level construct called "inline checks", which you can use to register a problem ad-hoc from within application code.
Problems registered by inline checks never show up in the admin dashboard, this is because when loading the dashboard, we run all realtime checks and look for problems. Because of an oversight, we considered inline checks to be "realtime", causing them to be run and clear their problem status.
To fix this, we don't consider inline checks to be realtime, to prevent them from running when loading the admin dashboard.
This PR introduces a basic AdminNotice model to store these notices. Admin notices are categorized by their source/type (currently only notices from problem check.) They also have a priority.
