In #26122 we promoted all problem checks defined as class methods on AdminDashboardData to their own first-class ProblemCheck instances.
This PR continues that by promoting problem checks that are implemented as blocks as well. This includes updating a couple plugins that have problem checks.
In AdminDashboardData we have a bunch of problem checks implemented as methods on that class. This PR absolves it of the responsibility by promoting each of those checks to a first class ProblemCheck. This way each of them can have their own priority and arbitrary functionality can be isolated in its own class.
Think "extract class" refactoring over and over. Since they were all moved we can also get rid of the @@problem_syms class variable which was basically the old version of the registry now replaced by ProblemCheck.realtime.
In addition AdminDashboardData::Problem value object has been entirely replaced with the new ProblemCheck::Problem (with compatible API).
Lastly, I added some RSpec matchers to simplify testing of problem checks and provide helpful error messages when assertions fail.
There are a couple of reasons for this.
The first one is practical, and related to eager loading. Since /lib is not eager loaded, when the application boots, ProblemCheck["identifier"] will be nil because the child classes aren't loaded.
The second one is more conceptual. There turns out to be a lot of inter-dependencies between the part of the problem check system that live in /app and the parts that live in /lib, which probably suggests it should all go in /app.
