GCI-32: Site header and session status revamp

Implement [GCI-32](https://issues.openmrs.org/browse/GCI-32) / [ID-69](https://issues.openmrs.org/browse/ID-69) ([Melange task](https://www.google-melange.com/gci/task/view/google/gci2014/5317001035644928)), revamping the site header. It now shows a blurb instead of a login button when there isn't an active session, and shows the profile image, username, and a logout button if there is an active session.