internal/ui: rework Mullvad peer list to be hierarchical

[DeedleFake]

Closes #141.

[DeedleFake]

I don't like how inefficient the Mullvad peer list update is. It has to create a map of slices, then turn that into a sorted slice of sorted slices. It's quite a few allocations and a decent number of sorts, too, though the sorts are pretty much all of lists that are probably less than 10 elements each, with one exception, so they're all quite fast.

Edit: To clarify why this is awkward, it's because it's doing that every single time Tailscale is polled, which is pretty much every five seconds or so. That's kind of awkward.