Warn when modifying prefix with differing auth src

The web UI now notifies users when trying to edit or remove prefixes
which has another authoritative source than 'nipap'. When removing a
prefix, the user needs to enter the name of the system as a
confirmation.

Part of #167
Warn CLI users regarding authoritative_source

CLI users are now warned before removing or modifying a prefix with
authoritative source other than NIPAP.

Fixes #167
Merge branch 'warn_edit_authsrc'