Groups, Suggestions: add support for @mentions UI in Group Activity screens.

This is achieved by adding support for a new `data-suggestions-group-id` property to the relevant textarea/input element.
See also r9242:9245 for incremental changes that support this new functionality.

Fixes #5934. Props m@rk, imath, DJPaul.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9276 cdf35c40-ae34-48e0-9cc9-0c9da1808c22