Group iterations by cadence on board add list form

What does this MR do and why?

This MR allows the user to add a board list by iteration (but showing the cadence associated with the iteration in the dropdown)

Repro

1. Feature.enable(:iteration_cadences)
2. Go to issue boards
3. Click create list
4. Select Iteration
5. Open the dropdown and select an iteration

Resolves part of #323653 (closed)

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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