(…and remove extraneous `stringify_keys` - enqueue already does
`deep_stringify_keys`)
The warning in question:
```
Deprecation notice: Jobs::RunProblemCheck was enqueued with argument values which do not cleanly serialize to/from JSON. This means that the job will be run with slightly different values than the ones supplied to `enqueue`. Argument values should be strings, booleans, numbers, or nil (or arrays/hashes of those value types). (deprecated since Discourse 2.9) (removal in Discourse 3.0)
```
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.
Doing the following renames:
Jobs::ProblemChecks → Jobs::RunProblemChecks
Jobs::ProblemCheck → Jobs::RunProblemCheck
This is to disambiguate the ProblemCheck class name, ease fuzzy finding, and avoid needing to use :: in a bunch of places.