Bugfix/fix orphaned certificate pivot rows

[Kroffie]

Add migration to cleanup old "ghost" pivot entries. Then add logic to detach users from certificates when the certificate gets deleted.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 40.79%. Comparing base (ee24ab9) to head (191e5d3).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
app/Certificate.php	50.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #522      +/-   ##
============================================
+ Coverage     40.77%   40.79%   +0.02%     
- Complexity      630      632       +2     
============================================
  Files            99      100       +1     
  Lines          2362     2368       +6     
============================================
+ Hits            963      966       +3     
- Misses         1399     1402       +3     

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[marijnvanderhorst]

Note that this defeats most of the purpose of using soft deletes for this model. After all, the certificate itself consists merely of a name and abbreviation. The most valuable certificate data lies in its relations, i.e., which users the certificate is/was assigned to.

In Laravel, soft deletes behave the same as actual deletes, unless the deleted records are explicitly requested using ->withTrashed(). Note that this allows us to keep the "pivot rows" of removed certificates while automatically hiding them when not requested. This makes restoring accidental deletes possible.

---

Observe that the certificate BelongsTo relation on the User model explicitly requests the certificates withTrashed:

esac.nl/app/Models/User.php

Lines 94 to 95 in f68854c

	return $this->belongsToMany(Certificate::class)
	->withTimestamps()->withTrashed();

In case we want to exclude removed certificates from the user's certificate overview (which IMO we do), we should simply remove the ->withTrashed() from that line. That would remove the need to detach on delete as well as the migration.