Create a way to scroll back up to the top of activity pages quickly

Problem: When the cards in the activity pages view are expanded, the view can get quite long, and the user would have to manually scroll all the way back up to the top. As a user, I'd like a way to quickly be brought back up to the top of the screen, so that I can navigate to other pages or use the search functionality on the page.

Acceptance criteria:
1. Give the user a way to scroll up to the top of the page no matter where they are on the page.
2. The icon to scroll should be recognizable, and should not block the view of annotation cards.

Start by reviewing the activity pages where expanded cards create long views, along with the existing navigation and search functionality. Add a recognizable control that returns the user to the top from any scroll position without obscuring annotation cards, then verify both acceptance criteria on a fully expanded page.

Written by the indexing model from the issue text.