Move "Next Step" to a custom field in Differential

Summary:
Fixes T9672. This was never turned into a custom field, for no particular reason. Convert it into one.

This is substantially similar to the existing "Apply Patch" field, which does the same thing (only shows a command).

We might rethink or remove this eventually (e.g., in a post-"Land Revision" world) but this makes it easier, at the very least.

Test Plan:

• Viewed a non-accepted revision (no hint).
• Viewed an accepted revision from a raw diff source (no hint).
• Viewed an accepted revision from Git (arc land hint).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9672

Differential Revision: https://secure.phabricator.com/D14367