Migrate mailers to use parametrized attributes

[MrSerth]

Related to #2959, we migrate all mailers to use parameterized attributes, since this should work better with testing.

[codecov]

Codecov Report

Attention: Patch coverage is 48.14815% with 14 lines in your changes missing coverage. Please review.

Project coverage is 69.77%. Comparing base (75b9297) to head (350a273).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
app/mailers/user_mailer.rb	59.09%	9 Missing ⚠️
app/controllers/comments_controller.rb	0.00%	2 Missing ⚠️
app/jobs/detect_exercise_anomalies_job.rb	0.00%	2 Missing ⚠️
app/controllers/request_for_comments_controller.rb	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2999   +/-   ##
=======================================
  Coverage   69.76%   69.77%           
=======================================
  Files         215      215           
  Lines        6834     6839    +5     
=======================================
+ Hits         4768     4772    +4     
- Misses       2066     2067    +1     

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[arkirchner]

Thank you!