Show a warning banner to administrators when there are unresolved setup warnings

Summary: We now issue only valid setup warnings, so we can let administrators know when we detect problems.

Test Plan:
Banner:

{F29568}

Created a fake issue; saw banner. No banner inside /config/. Resovled the issue, banner went away.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T2221

Differential Revision: https://secure.phabricator.com/D4493