Improve meta tags form usability

A starting point. It needs more work, but this is easier to keep track of than git stashes ;-)

Should the OG meta tags be merged into the two main groups? Maybe extend the groups to be topic-based, similarly to how they're grouped in Nodewords (6.x-1.14-beta2)?

I've committed the patch from #3 but further work needs to be done.

Leaving this as-is, we can open other tasks if needed.