Add an "ignore all" whitespace mode

Summary:
We used to have "ignore all", but it became "ignore most". It does not ignore in-line whitespace changes or whitespace changes in language where whitespace is marked semantic.

Add an explicit "ignore all" option.

Test Plan: Used "ignore all" to view a change with heuristically-detected semantic whitespace, the change was ignored.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T970

Differential Revision: https://secure.phabricator.com/D1865