Don't warn that workboard columns need a name when editing milestone columns

Summary: See https://discourse.phabricator-community.org/t/columns-must-have-a-name-while-editong-point-limit-on-milestone-column/2650. This check doesn't make sense for proxy columns, including milestone columns.

Test Plan: Added a point limit to a milestone column.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20448