Sidebar tags cards

[jywarren]

Fixes #6005 - replaces #6124 🎉

OK, I worked on this a good bit to get it to render well with many different combinations of tags, different screen sizes, etc. I hope this gets most of the edge cases, but it's not perfect... we may need to do some follow-up tweaks. In any case, here's what I'm seeing locally:

Mobile width initially:

And expanded:

Full width initially:

And expanded: