R1:ad3c94dd454f

Make "Show Context" persist rendering, whitespace, encoding, etc

Summary:
Ref T2009. Currently, we do not persist view parameters when making context rendering requests.

The big one is the renderer (1up vs 2up). This makes context on unified diffs come in with too many columns.

However, it impacts other parameters too. For example, at HEAD, if you change highlighting to "rainbow" and then load more context, the context uses the original highlighter instead of the rainbow highlighter.

This moves context…