Fix user can be born in the future (#327)

[codecov[bot]]

Codecov Report

Merging #329 (ea3913b) into master (a96f0c8) will decrease coverage by 0.01%. The diff coverage is 11.11%.

@@            Coverage Diff             @@
##           master     #329      +/-   ##
==========================================
- Coverage   51.43%   51.42%   -0.02%     
==========================================
  Files         198      197       -1     
  Lines        9680     9640      -40     
==========================================
- Hits         4979     4957      -22     
+ Misses       4701     4683      -18     

Impacted Files	Coverage Δ
prologin/users/forms.py	53.50% <11.11%> (-3.10%)	:arrow_down:
prologin/contest/views.py	38.70% <0.00%> (-0.58%)	:arrow_down:
prologin/homepage/views.py	28.26% <0.00%> (-0.32%)	:arrow_down:
prologin/contest/event_wishes_override.py
prologin/contest/forms.py	43.82% <0.00%> (+0.20%)	:arrow_up:

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[Elfikurr]

I believe that clean() should raise a ValidationError when encountering an error ? Especially if there is only one error, else we should use add_error() instead of self.error_class, if I'm reading the docs (badum tss) correctly.

[Nhqml]

IMO, the use of validators as suggested by @lportemo is a cleaner way to achieve our goal. It will also be more consistent if we decide to implement more validations steps in the future.

I believe we should instead focus on #331. Feel free to reopen this if you disagree.