Make "Ignore All" the default whitespace mode

Summary:
This mode was fixed in D174 by grglr and is the best mode for almost all changes
once nonbroken, so make it the default.

This is also the mode which takes advantage of the render cache.

Test Plan:
Loaded a revision, got "Ignore All" as the default mode, and revision rendering
wasn't silly/broken.

Reviewed By: tuomaspelkonen
Reviewers: grglr, jungejason, tuomaspelkonen, aran
CC: aran, tuomaspelkonen
Differential Revision: 229