Split Diffusion "view" preference into blame and color preferences

Summary:
We have this silly "view" preference which has a variety of silly values: "plain", "plainblame", "highlighted", and "blame", and then also "raw", which is magical. This is really just two flags: color on/off, and blame on/off (plus a separate mode for raw).

Express the code in terms of the flags and, e.g., get rid of the state transition tables we had before.

Test Plan: Viewed code in all four modes.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7046