This PR does some preparatory refactoring of scheduled admin checks in order for us to be able to do custom retry strategies for some of them.
Instead of running all checks in sequence inside a single, scheduled job, the scheduled job spawns one new job per check.
In order to be concurrency-safe, we need to change the existing Redis data structure from a string (of serialized JSON) to a list of strings (of serialized JSON).
This commit introduces scheduled problem checks for the admin dashboard, which are long running or otherwise cumbersome problem checks that will be run every 10 minutes rather than every time the dashboard is loaded. If these scheduled checks add a problem, the problem will remain until it is cleared or until the scheduled job runs again.
An example of a check that should be scheduled is validating credentials against an external provider.
This commit also introduces the concept of a `priority` to the problems generated by `AdminDashboardData` and the scheduled checks. This is `low` by default, and can be set to `high`, but this commit does not change any part of the UI with this information, only adds a CSS class.
I will be making a follow up PR to check group SMTP credentials.
