Container task for implementing subprojects, described in T3670.
- [X] 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.
- [X] You can reorder subproject and milestone columns, but should not be able to.
- [X] Renaming subproject and milestone columns probably works but should not.
- [ ] Milestone/subproject column headers should be linked to their workboards.
- [X] 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.