Allow underscore override per RR

[mzagrabe]

I'd like to add one or two RR's to a domain, but I don't want to toggle the config option:

ALLOW_UNDERSCORES_IN_DEVICE_NAMES

Instead, I've added a new config option:

UNDERSCORES_IN_DEVICE_NAMES_OVERRIDE_ALLOWED

that if set will display a checkbox on host.html when adding a record from management/zone_tasks.html. Checking the checkbox will allow that record to contain underscores.