Make it easier to ignore setup issues

Summary:
Ref T4331. Ref T5968. Users sometimes have trouble figuring out how to ignore issues. The option is a bit hard to spot, especially if you aren't familiar with interfaces yet.
Make it a button on the issue page itself instead.

Test Plan:
Normal issue:

Ignored issue:

{F199226}

Fatal issue:

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T4331, T5968

Differential Revision: https://secure.phabricator.com/D10420