Fix a couple of column editing issues

Summary:
Ref T10349.

• Don't show subproject columns on "Manage Board".
• Fix "Edit Column" for milestone columns (allows you to set points, but not rename).

Test Plan:

• Viewed "Manage Board" on a project with subprojects.
• Edited a milestone column and set a point limit.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10349

Differential Revision: https://secure.phabricator.com/D15338