Don't auto-link @mentions that appear inside the context of links

This prevents illegitimate double-linking when text like
href="mailto:foo@bar.com" or <a href="http://twitter.com/bar">@bar</a> happen
to match a BP username like @bar.

Fixes #4680

Props ninnypants

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7890 cdf35c40-ae34-48e0-9cc9-0c9da1808c22