at-mentions: overhaul the mentions implementation for Private Messages and Groups (admin) components.

Previously, these components had seperate implementations of username auto-suggestions, which weren't
reusable outside of where they'd been built, nor easily extended by developers, or other plugins.
This change creates a central API for auto-suggestions, currently only for usernames, but easily
extensible for other kinds of auto-suggestions in the future (i.e. group names, or #hashtags).

See #3278

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8557 cdf35c40-ae34-48e0-9cc9-0c9da1808c22