VOL-71: Handle Joomla's permissions issues gracefully

The "register to volunteer" permission that CiviVolunteer adds to CiviCRM is really a bonus feature, not a must-have feature. The way we currently handle a noncooperative CMS is suboptimal. 

This patch establishes a way to determine whether the installation supports extension-defined permissions. If so, CiviVolunteer:
- installs the permission
- enforces the permission
- documents the permission in-app

Otherwise, it does none of these things. In other words, we let everyone access the sign-up form, and we take away the admin's ability to restrict access.