Feature: Add site to block referer

[molitar]

This header works better than the older referrer control but it's missing an easy to add option to add site to be blocked. In Referrer Control it was a context menu to add it so did not even need the extension to even show on the toolbar. This one requires always showing on toolbar as I could not other way to start it and manually typing in to add a site to it.

1. Easy option to add site to block referer
2. Give management capability to the Add-Ons page so do not have to show in a crowded toolbar window
3. Import configuration - give option to not delete current configuration so a single site import can be done