Allow rotations to gracefully switch between parameters

What does this MR do?

Related issue: #262862

This MR adds logic for when a user takes manual action on a rotation (create or edit) and we want to have the effect take place immediately on the saved shifts.

• For new rotations which have already "started", we will simply trim the first shift to start at the creation time of the rotation.
• For edited rotations (!55258 (merged)), the old parameters and the new parameters may conflict, leading to errors in saving shifts. This is prevented by cutting the existing oncall shift short, and immediately starting a new one partway through.
• If there is no existing saved shift or no new shift, as might happen when a rotation has active periods, nothing will happen.

Screenshots

Example: I created a rotation at 8:15pm which started before today called "Cut me off yo". You can see the start time of the new shift is trimmed to the creation time.

Does this MR meet the acceptance criteria?

Conformity

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