Allow "Repository Automation" to be configured for repositories

Summary:
Ref T182. This allows you to assign blueprints that a repository can use to perform working copy operations. Eventually, this will support "merge this" in Differential, etc.

This is just UI for now, with no material effects.

Most of this diff is just taking logic that was in the existing "Blueprints" CustomField and putting it in more general places so Diffusion (which does not use CustomFields) can also access it.

Test Plan:

• Configured repository automation for a repository.
• Removed repository automation for a repository.

Reviewers: chad

Reviewed By: chad

Subscribers: avivey

Maniphest Tasks: T182

Differential Revision: https://secure.phabricator.com/D14259