Group roadmap view of epics

See latest ongoing designs in comment thread.

Design

• Navigate to the roadmap page in a group.
• That roadmap page shows all the epics of that group.
• How to handle permissions? TBD depending on permissions model of epics. Possibly you can't see all epics depending on permissions.
• Latest mockups: https://gitlab.com/gitlab-org/gitlab-ee/issues/3293#note_40977850

Issue states and timestamp/date mapping to epics

• All issues in epic are closed: Epic is complete.
• All issues in epic are open: Epic is not started.
• At least one issue in epic is in progress: Epic is in progress.
• Epic planned start date: From epic field.
• Epic planned end date: From epic field.
• Epic actual start date
  • If all issues are open, then actual start date is null. I.e. we ignore the fact that some issues might have went into in progress and then gone back to open.
  • Next, find the earliest non-null enter-in-progress timestamp. If it exists, this is the actual start date.
  • Last, take the earliest non-null closed_at timestamp. This is necessary if all issues just switched directly from open to closed.
• Epic actual end date
  • If there are any open issues or in-progress issues, the actual end date is null.
  • Otherwise, look at all closed_at timestamps. Take the latest one. That is the epic actual end date.
  • Some legacy issues don't have closed_at timestamps. If all closed issues in the epic here don't have a closed_at timestamp, just take epic actual end date as epic planned end date.

Not in scope

• Creating and managing roadmaps (https://gitlab.com/gitlab-org/gitlab-ee/issues/3481) is not in scope. In this first iteration, there will only be one roadmap view that you see by default when you navigate to the roadmaps screen.