vpn: add support for custom keys

Custom keys are created by admin and specify networks directly,
bypassing AD permissions. They are intended to join managed devices
into networks where users are not allowed to create keys themselves.

Also comprehend a set directly.

web/templates/vpn/wg-fri.conf

@@ -1,5 +1,5 @@
 [Interface]
-# {{ timestamp }} {{ current_user['username'] }} {{ name }}
+# {{ timestamp }} {{ user }} {{ name }}
 # PublicKey = {{ pubkey }}
 PrivateKey = # paste private key here
 Address = {{ ip }}{% if ip6 %}, {{ ip6 }}{% endif %}