Integrate db checks from wp-audit-tool into mu-plugin

[jazzsequence]

This project is no longer maintained but has a lot of valuable tools that appear to overlap with things that the mu-plugin is currently doing. Pulling more of these things into the mu-plugin would be ideal.

Based on input from Professional Services, wp-audit-tool is primarily used as a standalone tool used to get useful details for prospective customers and migrations to Pantheon. As such, there's a lot we shouldn't include (as it would be pointless or irrelevant) and it should continue to exist as a standalone plugin for those cases. However, the DB checks could still be valuable as a site health check.