Prefill iteration date with sensible value

What does this MR do and why?

When creating new iterations in manual cadences:

1. If cadence has iterations, use day after the last cadence
2. Otherwise use the date from the cadence

Screenshots or screen recordings

How to set up and validate locally

1. Enable the feature flag

   Feature.enable(:iteration_cadences)

2. Visit any group and create a manual iteration cadence (uncheck 'Automatic' checkbox)
3. Click the ellipsis > Add iteration
4. Date field should be pre-filled with cadence start date
5. Save iteration, add a second one
6. Date field should be pre-filled with day after the first iteration's due date

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.