Improve "Ignore" action for setup warnings

Summary:

• The dialog has no body text.
• It uses an "Ignore" link because it predates action items.

Test Plan:
{F49894}
{F49895}
{F49896}

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D6433