Calendar showing issues on issue due dates

• At group level, show a calendar.
• Show all issues in that group, with a due date.
• Show issues on the calendar with that due date.
• In the future, have additional filtering / saving config. But for first iteration, show all issues with a due date.
• We need further UX design to determine what should be in each issue entry in a given calendar day.

Original description

### Problem to solve

It is very hard to have a long-term overview of issues because there is only a list and the calendar exports lacks support for longer planning timeframes (also see gitlab-ce#54133).

Further details

I want to be able to have a calendar-based overview of all issues planned in the past and future. It would be nice to have this kind of overview both on a project and a group level.

Proposal

An alternative “calendar view” should be implemented in addition to issue boards. A nice way to do this has been implemented by Trello.

This would also make a lot of sense in combination with gitlab-ce#52542.

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