Deprecate "Track Only" in the Diffusion UI

Summary:
Depends on D20432. Ref T13277. Fixes T12967. Removes some "Track Only" hints and warns that the feature is deprecated in favor of "Permanent Refs" and "Fetch Only".

(This "fixes" T12967 by mooting it.)

Test Plan: Viewed "branches" sectino of the manage UI, edited "braches" section of a repository.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13277, T12967

Differential Revision: https://secure.phabricator.com/D20433