Hide Land to GitHub

Summary:
We don't want to support this right now, so disable it, similar to "Land To Hosted" feature.
Keep to the code as an example for advanced installs.

Ref T182, T8313

Test Plan: load a diff.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley

Maniphest Tasks: T182, T8313

Differential Revision: https://secure.phabricator.com/D13022