Simplify CSS for overflow.

This PR is a final take on the overflow of the left-adjusted sidebar. It should allow nice rendering of all pages in a narrow browser window. Fixes bullet-point issue noticed by @ximenesuk (https://github.com/openmicroscopy/sphinx_theme/pull/15#issuecomment-49849174). 

The final thing to fix after this get merged is the LDAP doc page (shorten the paragraph headings).