Handled overlaping event bugs with better comparison logic

[nmhossain02]

Addresses #71

In this PR:

• I extracted the giant mapping function before the final return statement in retreat/[id].tsx for better readability (imo)
• I turned a <= to a < to fix the falsely overlapping events bug
• I introduced a function to turn time-of-day strings into minutes for more reliable comparison
• I sorted the eventsByDay arrays to catch more edge cases with overlapping events
• I tried to simplify some logic in retreat/[id].tsx:131; it seems to be working fine for my testing, but it's worth a look

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[nmhossain02]

non-null assertion operator goes hard