Schedule iterations on cadence start day

Requirements #354977 (comment 911380741)

1. All scheduled iterations need to start on the same day as the cadence start day.

2. Start date of cadence cannot be edited once the first iteration has started.

3. Start date of cadence can be edited anytime before the first iteration has started. In this case, the dates of the iterations, if any exists in the cadence, should be re-aligned.

   Example.

   1. If I create a cadence with a start date of Jun 6
   2. It is May 1 and I update the cadence to start the cadence on May 1
   3. The two iterations in June would have their dates changed to May 2-8 and May 9-15 respectively