Container task for implementing subprojects, described in T3670.
- Milestone icon in the workboard column header has whack spacing.
- If you edit a task and change which milestone it's in, we don't move it to the proper column.
- You can reorder subproject and milestone columns, but should not be able to.
- Renaming subproject and milestone columns probably works but should not.
- Milestone/subproject column headers should be linked to their workboards.
- Add "Initial Members" on project create and drop autojoin behavior to reduce the number of "tag" projects with the author as a member.
- "Subprojects" profile menu item should be hidden for milestones.