R1:4e181a5611fb

Clean up browse/history links in Diffusion

Summary:
Fixes T9126. In particular:

- Add "Browse" links to all history views.
- Use icons to show "Browse" and "History" links, instead of text.
- Use FontAwesome.
- Generally standardize handling of these elements.

This might need a little design attention, but I think it's an improvement overall.

Test Plan:
- Viewed repository history.
- Viewed branch history.
- Viewed file history.
- Viewed table of contents on a commit.
- Viewed merged…