Clarify sprint columns and adapt them to new roles [MNG-605]
This MR:
- Removes the "Ready for Merge" column. Issues would often get forgotten in that column, and this is also an attempt to make our process simpler. When a review is complete, leave the issue in "Internal Review" until the assignee moves it to either "Merged" (if no more review is needed) or "External Review/Blocker" (if we need to wait for another review, or we cannot merge it until some other thing happens first).
- Renames the "Delivered" Column to "Deployed & Delivered" to make deployment status formally indicated somewhere.
- Allows assignees to move their own tickets into "Deployed & Delivered"
- Clarifies what the Sprint Manager should do when reviewing tickets in "Deployed & Delivered" and moving them to Done.
Note: Braden and/or Xavier will still review all the tickets in "Done" before the end of the sprint, but that won't be indicated with its own column anymore.