schedule/rotation: fix endtime calculation bug

[mastercactapus]

• Identified the issue which this PR solves.
• Read the CONTRIBUTING document.
• Code builds clean without any errors or warnings.
• Added appropriate tests for any new functionality.
• All new and existing tests passed.
• Added comments in the code, where necessary.
• Ran make check to catch common errors. Fixed any that came up.

Description:
This PR fixes a bug in Rotation.EndTime that would return an incorrect value causing an infinite loop in some cases.

Additional Info:
When it comes to rotations we use a single timestamp and timezone to denote when the "rotating" begins.

For weekly and daily this is how a rotation at 8 pm keeps its meaning across timezone boundaries. Additionally, changing the timezone of the rotation won't "lose its place" because it's evaluated as "what time was it, at this moment, in that timezone".

When dealing with rotations we have one method that does most of the heavy lifting EndTime. It takes a single argument, t, any timestamp, and will output the time at which a rotation shift would be over, for whatever shift is active at t.

There is also a StartTime method, which just calculates the EndTime and subtracts 1 shift from it.

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When calculating schedules for who is on-call at any given time, there is a method that uses this, named UserID, which takes a timestamp t and returns the user who would be active at that time.

To do this, it will walk the rotation forward or backward to find the correct shift. It will advance the rotation until EndTime is after t or roll it back until StartTime is before t.

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The loop bug found was that if the rotations configured Start time is after the t argument to EndTime, it will return the Start value.

When mixed with the on-call code, that meant that the EndTime would be in the future, and the StartTime would always be 1 shift length behind EndTime. This caused the UserID method to infinitely "rollback" the rotation since every time it rolled back it would actually get the same value.

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Viewing a schedule where an assigned rotation's Start value is > 1 shift length past the current time would cause it to loop infinitely.

[m17ch]

LGTM 👍