Set a blank icon for missing apps. Scale up small icons

Anon AppVM like anon-whonix needs an anon-gateway like sys-whonix
to provide a correct level of anonymity and privacy.
This commit adds a warning when a user try to connect a
VM with the tag anon-vm to a Net Qube that does not have the tag
anon-gateway

* origin/pr/411:
  add a test for warning message when changing anon-vm netvm
  add a warning box when connecting an anon vm to a non-anon gateway

Pull request description:

Fixes QubesOS/qubes-issues#8551

Anon AppVM like anon-whonix needs an anon-gateway like sys-whonix to provide a correct level of anonymity and privacy. This commit adds a warning when a user try to connect a VM with the tag anon-vm to a Net Qube that does not have the tag anon-gateway

For tests to pass, QubesOS/qubes-core-admin-client#343 must be merged.

* origin/pr/418:
  Show qube label icon in backup selection

* origin/pr/419:
  Improve and fix Settings App icons

* origin/pr/422:
  anon gateway warning box on main qubesmanager page

Pull request description:

Solves:

> Please re-open.
>
> [A user reported](https://forums.whonix.org/t/re-stop-users-from-changing-their-anon-whonix-net-qube-to-sys-firewall-to-avoid-ip-leaks/17255/7):
>
> I remember looking at this and patching my Qubes Manager temporarily and it failed to account for switching NetVM in the basic view. The pop up only shows in the settings tab, meaning this is only half solved
>
> This should be verified and reported before 4.3 is out, I have not checked the code again

 _Originally posted by @adrelanos in [#8551](QubesOS/qubes-issues#8551 (comment)