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.

In Joomla's case, since CiviCRM-extension-defined permissions are not rendered in the CMS's permissions administration UI, the "register to volunteer" permission cannot be assigned to users except with database hacking. The result (for non-hackers, anyway) is that permission to use the form is denied to everyone (except possibly the admin user).

Instead of shipping a broken bonus feature, we should remove it altogether for Joomla. In other words, let everyone access the sign-up form, and take away the admin's (broken) ability to restrict access.

Note re user/admin doc: CiviVolunteer's README and in-app doc refer to the "register to volunteer" permission. This in-app doc should be made to appear conditionally based on the user framework, and the README should be updated to reflect these changes.