Convert network page to use WTForms

* Regexes on frontend and backend for each field in the network form are the same, just like edit form
* View logic works pretty much the same as the edit container view, we loop over each field and check if the field needs saving
* Only restart network, if something is actually changed (this is new)
* Updated forms/__init__.py it so both forms can be imported from lwp.forms, this is not essential but makes importing the forms nicer

 There is still a bug (that was there before), where network settings are read from /etc/default/lxc-net (the correct place), but written to /etc/default/lxc (this still needs fixing)