install qubes-usb-proxy in default templates or change documentation to reflect

[mfc]

Qubes OS version:

R4.0

Affected component(s):

default templates: Debian 9, Whonix-workstation

Fedora 26 template has it installed by default.

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Steps to reproduce the behavior:

attempt any activities that require qubes-usb-proxy and get error that it is not installed (or get silent fail if using GUI).

Expected behavior:

qubes-usb-proxy is installed in these default templates or change documentation to reflect this. In the documentation it says: "qubes-usb-proxy should be installed by default in the template VM".

Actual behavior:

Debian 9 and Whonix-workstation do not have it installed by default.

Related issues

This is an example where better tracking of installed packages in default templates may be helpful.