R1:bec21d80a584

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…