Don't try to import proxy columns

Summary:
Fixes T10346. You finally wrung a clue out of the reporter and I think I figured this out.

Here's the bug:

• Create a project with a workboard and subprojects/milestones.
• Create a new project, import columns from the first project.
• We incorrectly import empty columns for the subprojects/milestons.

Instead, skip proxy columns during import.

Also, allow "hide column" to continue on missing fields, so columns with no name can be hidden.

Test Plan:

• Did the stuff above.
• Workboard no longer populated with a bunch of "Unnamed Column" columns.
• Hid several "Unnamed Column" columns.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10346

Differential Revision: https://secure.phabricator.com/D15265