try making page buttons "sticky" like toc

**DO NOT MERGE**

This is for @openjck, @darkwing, and @stephaniehobson to see something I want to try with an Optimizely A/B test: i.e., if we can "stick" the page buttons (especially the edit button) to the top of the window as the user scrolls. This should make it easier to edit, and so we should see an increased number of clicks on the "Edit" button.