Fixes #7462 - new UI for network interfaces

Editing network interfaces moved into a modal window. Some screenshots of the new UI are at:
https://tstrachota.fedorapeople.org/screenshots/nics_ui/

Bits of host form related to NICs are initially hidden and copied into a modal window on edit button click. This allows reusing validation error notices rendered by rails.
nitpick: could you please use interfaces_invalid without @ so we don't risk any conflict in future?