This adds two links at the top of the home page that go to different
"views" - one includes only topics related to the coronavirus, while the
other filters out all coronavirus topics.

For the purposes of these views, a "coronavirus topic" is one that is
either in the ~health.coronavirus group, or has coronaviruses.covid19
tag.

This is very hacky and will only work on the main tildes.net site due to
hardcoded group name and tag. I'm okay with that.