Allow activity filter dropdown to be wider.

This prevents the filter text from breaking to a second line in themes with
slightly larger default font sizes, such as Twenty Fifteen.

Props colorful tones.
Fixes #6075.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9252 cdf35c40-ae34-48e0-9cc9-0c9da1808c22