Allow the use of existing calendars for deck boards

When a new board is created, automatically a new calendar is created.

Let's say a working group has already set up a group-specific calendar called "Workgroup 1" and later creates a board with the same name. In this case, a new calendar named "Deck: Workgroup 1" is created and you will see two calendars: "Workgroup 1" and "Deck: Workgroup 1" in the Calendar app where users belonging to the group may not add/maintain user specific appointments to the calendar created by the Deck app.

At least from my point of view, this is a bit confusing as a workgroup only needs one single calendar where participants can maintain the group specific appointments and the deck app can manage the due dates for the board cards related to that workgroup.

I suggest that a user creating a new board can optionally select an existing calendar for that board. If no existing calendar is selected, the application will create a new calendar as it does now. In addition a calendar automatically created by the Deck app could allow users to maintain manually created appointments.

Also, a user may decide at a later date to use an existing calendar for a board. In this case, the existing due dates managed by the Deck app must be moved to the newly connected calendar, the board-calendar-connection must be changed to the newly selected calendar and the calendar created by the Deck app must be deleted.

No files or tests are named in the issue. Start by tracing board creation and the existing Deck calendar connection, then identify how due dates are stored and synchronized; done means users can select or later switch to an existing calendar, due dates move correctly, and an automatically created calendar can be removed without losing appointments.