Move revision header warnings into custom fields

Summary:
Ref T5495. We currently show one warning in revision headers, about not having any reviewers.

I want to add a second warning (for missing Legalpad signatures). At least one install would like to add custom warnings (see T5495) which are so specific that we can't reasonably cover them in the upstream.

Generalize these header warnings by moving them to CustomField, so I can implement the Legalpad stuff without making a mess and the install in T5495 can use an extension.

Test Plan:
Hit all three header states, they look exactly like they did before this change:

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5495

Differential Revision: https://secure.phabricator.com/D9793