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 changes on a merge commit.
• Viewed a directory containing an external.
• Viewed a deleted file.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9126

Differential Revision: https://secure.phabricator.com/D14096