Provide a web interface to view raw source text in Differential

Summary:
Add links to the 'standalone view' to grab the raw source text. I think this
operation is rare enough that it's okay to hide it like this. I changed
'Standalone View' to 'View Standalone / Raw' to improve discoverability.

This also fixes the broken Standalone View links in Diffusion by no longer
rendering them.

Test Plan:
viewed old and new sources for a changeset

Reviewed By: tuomaspelkonen
Reviewers: tuomaspelkonen, jungejason, aran
CC: aran, tuomaspelkonen
Differential Revision: 243