Use selected VRF in web ui for new prefixes

After my fixes to the VRF panel, a few things broke in the web UI when
trying to add a new prefix. The VRF wasn't honoured at all - all prefix,
except those added from a pool, would end up in the default VRF. That
has been fixed both for manuall adding prefixes as well as when using
from-prefix.

Fixes #477.
Merge branch '477-fix-web-from-prefix'