webui: Change networking mode to vlan when using linuxbridge

This is a regression in the bootstrap port.

We also remember the non-linuxbridge value to switch back to it if
people try different options before saving.
webui: Remove duplicated javascript bits

These changes will already be done through the call to
networking_mode.trigger('change');
Merge pull request #22 from vuntz/fix-js

webui: Change networking mode to vlan when using linuxbridge