Use OIDC SSO provider instead of deprecated cern one

[nothingface0]

Updating django-allauth config to use the new SSO which is OIDC-based. There was no need to update the cern provider in django-allauth, as the openid_connect adapter works just fine.

See discussion in this django-allauth PR.

Extra steps that have to be done after deploying:

1. "Migrate" the existing SocialAccount entries as follows:
   1. Run python manage.py shell.
   2. Run:

   from allauth.socialaccount.models import SocialAccount
   sa = SocialAccount.objects.filter(provider="cern")
   for s in sa:
   	if 'username' in s.extra_data:
   		s.uid = s.extra_data["username"]
   		s.save()

2. Extra env vars that need to be added:
   1. CERN_SSO_REGISTRATION_CLIENT_ID
   2. CERN_SSO_REGISTRATION_CLIENT_SECRET

[codecov-commenter]

Codecov Report

Merging #239 (078bca4) into develop (91b5665) will decrease coverage by 0.01%.
The diff coverage is 83.33%.

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@@             Coverage Diff             @@
##           develop     #239      +/-   ##
===========================================
- Coverage    69.35%   69.34%   -0.01%     
===========================================
  Files          164      164              
  Lines         5988     5996       +8     
===========================================
+ Hits          4153     4158       +5     
- Misses        1835     1838       +3     

Impacted Files	Coverage Δ
users/signals.py	95.31% <78.57%> (-4.69%)	⬇️
users/tests/test_users_signals.py	100.00% <100.00%> (ø)

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