R1:2f054edfa22e

Hide milestone columns when milestone is archived

Summary: Fixes T10310. This replaces the "Hide Column" / "Show Column" option for milestone columns with one that archives/unarchives, and hides milestone columns when the milestone project is archived.

Test Plan:
- Hid and unhid a normal column (got normal dialogs).
- Hid and unhid a milestone column (got "archive project" dialogs, underlying project archived/unarchived, column vanished).

Reviewers: hach-que, #blessed_reviewers, chad

Reviewed By…