Make viewer header sticky (#772).

Make viewer header position:fixed when scrollTop is not 0. Handle
offset for body when this happens via padding-top. This is all
handled in JS rather than partly in CSS and partly in JS.
Cross-tested against issue #684; works ok.
Merge branch '772-viewer-header' of https://github.com/patternleaf/atlas

Addresses #772

Conflicts:
	static/js/viewer/views.js
Update changelog to reflect #772 changes