Improve `<label>` markup for mesage search form.

* Associate the label properly with the labeled element.
* Add bp-screen-reader-text class for better consistency with other similar text in BP templates.

Props hnla.
Fixes #6010.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9240 cdf35c40-ae34-48e0-9cc9-0c9da1808c22