Improve UI for Mullvad exit nodes

[felschr]

Mullvad exit nodes when enabled clutter the list of nodes & exit nodes.

I propose a few changes:

• remove Mullvad exit nodes from normal "nodes"
• group Mullvad exit nodes by country and/or city in "exit nodes"
• add search, if possible

For reference the UI used by tailscale web looks like this:

On iOS they have got this UI:

https://github.com/maxgallup/tailscale-status/assets/3314323/37a002b3-97dc-44c2-b8fa-2a34badd9444