Refactor authz web user interface to have common code and templating

[rgrp]

Currently repeat the same template and code across Package Authz, Group Authz, and Authz Group authz.

Having now implemented a new, cleaner setup in ckanext-admin we should port this back into core.

• Common template code (checkbox template)
• Logic code (or just common code) for wiring into authz system
• Look for all places thoroughout the system where usernames, authzgroups or groups need to be typed into boxes, and make sure that they auto-complete appropriately.

Will also deliver a significant improvement in the form of ajax user lookup.