R1:6039ca6fb554

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…