Create option for differential custom fields to display warning on accept

Summary:
This adds support to differential fields to display warnings before a revision
gets accepted. Since lint and unit are differential fields, the code for their
warnings was moved into their respective field specification classes, so there
is only one code path for warnings (lint, unit, or custom).

Test Plan:
Select 'Accept' on a revision with lint/unit warnings and see messages appear
like they used to. Change it back to 'Comment' and they go away. Repeat with
a revision without lint/unit warnings and see no warnings appear. Checked
darkconsole to see no errors due to this.

Reviewers: jungejason, epriestley, vrana

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2363