GCI-33: Revamp site footer

Revamp the site footer, as per [GCI-33](https://issues.openmrs.org/browse/GCI-33) / [ID-70](https://issues.openmrs.org/browse/ID-70). I also added some top margin to the about view to distance it from the main content. 

[Melange task](https://www.google-melange.com/gci/task/view/google/gci2014/4893200305094656).