[UX] Consolidate database-related information in the status report page, and add some basic db usage metrics

This is the second split-off from #5858, and a sibling issue to #5863.

In #5858 we considered adding some system metrics to the status page, to help site admins/owners get a better understanding of the minimum-recommended and minimum-possible specs required for a server that would run their site (when they are looking to move their locally-developed site to a live server, or when switching hosting providers).

I've noticed some code repetition and certain things that could be consolidated. For example, things like the "MySQL Database 4-byte UTF-8 support", and "Database updates" entries, which I think would be better placed under the "more" toggle in the main "MySQL Database" entry (where we show the db version). That, plus some other minor tweaks.

Also, I wanted to split this from the rest of the things we considered (such as PHP memory limit and files directory size), in order to not hold back one thing while discussing/bikeshedding the others.