Display file name in <title> for easier browsing of code

Summary:
Having the filename in the <title> will make it easier to browse through many
many tabs of diffusion

Test Plan:
Open up a file or directory in diffusion and ensure that it shows only the file
name.

Reviewed By: tuomaspelkonen
Reviewers: epriestley, tuomaspelkonen, jungejason
Commenters: epriestley
CC: epriestley, aizatto, tuomaspelkonen
Differential Revision: 150