[BB-3173] Fix vacations for negative timezones

[Agrendalath]

This fixes a bug with calculating vacations for negative timezones from #59. That PR did not invert the division for negative timezones, so these cases were not being classified correctly:

1. Negative timezone, non-overlapping day, vacations at the last day of the active sprint. These were counted as a part of the next sprint instead of the active one.
2. Negative timezone, non-overlapping day, vacations at the last day of the next sprint. These were counted as a part of the future next sprint instead of the next one.

Testing instructions:

1. Check that the tests are passing.

[0x29a]

👍

• I checked that tests are passing.
• I was rereading diff until I understood what caused bug and why change fixes it.
• Includes documentation.