Delete old Maniphest edit controller

Summary:
Ref T9908. No more callsites. Also:

• Phurl a couple of documentation URIs.
• Get rid of "task:" in the global search since it doesn't really make sense anymore.

Test Plan: grep, edited/created tasks.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9908

Differential Revision: https://secure.phabricator.com/D14716