R1:a1516fefb67a

Fix an issue where "Import Columns" could fail on a board for a project with milestones

Summary:
See PHI1025. When you "Import Columns", we test if you're trying to import into a board that already has columns. However, this test is too broad (it incorrectly detects "proxy" columns for milestones as columns) and not user-friendly (it returns 400 instead of a readable error).

Correct these issues, and refine some of the logic around proxy columns.

Test Plan:
- Created a project, A.
- Created a…