Fix some issues in free/busy suggestions

[GVodyanov]

Fix #6162

Improve the slot suggestions and make the algorithm more understandable

[codecov]

Codecov Report

Attention: Patch coverage is 85.41667% with 7 lines in your changes missing coverage. Please review.

Project coverage is 23.54%. Comparing base (343fe47) to head (aac9479).
Report is 17 commits behind head on main.

Files with missing lines	Patch %	Lines
src/services/freeBusySlotService.js	87.23%	6 Missing ⚠️
src/components/Editor/FreeBusy/FreeBusy.vue	0.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main    #6175      +/-   ##
============================================
- Coverage     23.73%   23.54%   -0.19%     
  Complexity      454      454              
============================================
  Files           247      247              
  Lines         11731    11702      -29     
  Branches       2223     2213      -10     
============================================
- Hits           2784     2755      -29     
  Misses         8627     8627              
  Partials        320      320              

Flag	Coverage Δ
javascript	15.07% <85.41%> (-0.26%)	⬇️
php	59.53% <ø> (ø)

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[GVodyanov]

For people reviewing, this is how the suggested slots should work:

• If the time you set for your event is free, it will be suggested, without being rounded (e.g. I set the time to 11:24-12:56, there are no blocking events in that period, that period will be suggested)

• Aside from suggesting the current time if free, it will look at all the events found by the free busy service, and see if everyone is free immediately after. So say two people are unavailable until 08:00 and 09:00, immediately after 08:00 is invalid, but immediately after 09:00 is.

• The time after the events will also be rounded to the nearest quarter-hour (so :00, :15, :30, :45), but always rounding up. There is always at least a minute of break between events, so say you have a blocking event ending at 13:00, the next event will be suggested at 13:15. If that blocking event were to end at 13:14, the next event would still be suggested at 13:15.