Cache the jq('#whats-new') object for improved performance

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7194 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Unset and reset prefilled @username in #whats-new field

This ensures that the cursor will be placed after @username on focus, instead
of at the beginning.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7195 cdf35c40-ae34-48e0-9cc9-0c9da1808c22