R1:4ec31ef75c3b

Provide basic capabilities to make Differential column width flexible

Summary:
- Make wrap width settable in PHP.
- Dynamically generate max-width based on configurable maximum width.
- Constrain non-diff elements to standard width.
- Provide a configuration setting.

Test Plan:
Set various things to 100 / 120, as far as I could tell everything seemed to
render sensibly? This should have no effect on 80-col changes.

Reviewed By: jdperlow
Reviewers: jdperlow, tuomaspelkonen, jungejason, aran
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