Adds popup manager

Adds logic and styles for templated popup manager section that works
for the About, Contribute, and Feedback links in hte header for now.
Fixes popup issue with Mobile Safari

mousedown event handler attached directly to document wasn't working in
Safari Mobile, so this attaches it to the #content element instead,
which fills the whole body area.
Fixes #152 - corrects inability to dismiss popups on Mobile Safari

Refactors terminology popup manager and attaches close handler to
#content element instead of the document, which wasn't being recognized
on Mobile Safari.