Config option to ignore setup issues

Summary: T2381

Test Plan:
Include existing setup issues in the ignore config option,
reduces the number of setup issues in the status bar, moves ignored
issues to the bottom of the list, and marks them as ignored.

Also include a string corresponding to no setup issue, and verify that
application does not break.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5072