First pass enhancements to `BP_Members_Admin`

* Introduce extended profiles to `/wp-admin/user/` dashboard.
* More robust single-site, multi-site, multi-network support, with access to extended profile data at all appropriate levels based on installation type, current user role, etc...
* Add post-row checks to ensure correct administrative access to other user's data.
* Allow super admins to edit any other user, always.
* Allow users to edit their own extended profiles.
* Various verbiage changes for improved clarity and workflow.
* Some monolithic functions broken down into smaller private methods.

There is more to do here, including unit test coverage to confirm no unintended user-data leakage or accidental capability escalation is occurring, testing with more esoteric WordPress installation types, etc...

Props imath for the original patch. See #5623.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8441 cdf35c40-ae34-48e0-9cc9-0c9da1808c22