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.
This was originally introduced in #26071, but that PR was closed, because the requirements changed. This PR lifts only the relevant parts, since they are a prerequisite for the new admin notice system.
As part of problem checks refactoring, we're moving some data to be DB backed. In this PR it's the tracking of problem check execution. When was it last run, when was the last problem, when should it run next, how many consecutive checks had problems, etc.
This allows us to implement the perform_every feature in scheduled problem checks for checks that don't need to be run every 10 minutes.
Ted Johansson