Fix overlapping classes bugs

[LuisDuarte1]

The overlapping lectures are not completely fixed, because the desired behavior will require a more deep refactor. Now if a overlapping lectures is from a group of classes, the end time will be equal do the start time. I'll create a issue for a more in-depth refactor

Review checklist

• Terms and conditions reflect the current change
• Contains enough appropriate tests
• If aimed at production, writes a new summary in whatsnew/whatsnew-pt-PT
• Properly adds an entry in changelog.md with the change
• If PR includes UI updates/additions, its description has screenshots
• Behavior is as expected
• Clean, well-structured code

[codecov]

Codecov Report

Merging #973 (e88317d) into master (a721c1a) will increase coverage by 1%.
Report is 3 commits behind head on master.
The diff coverage is 45%.

Additional details and impacted files

@@          Coverage Diff          @@
##           master   #973   +/-   ##
=====================================
+ Coverage      22%    22%   +1%     
=====================================
  Files         143    143           
  Lines        4357   4368   +11     
=====================================
+ Hits          948    953    +5     
- Misses       3409   3415    +6