Enhance Authentication

[temparus]

• Fixes the LDAP sync issue for users which are not enrolled at ETH anymore.
• Allows for ETH members to authenticate against LDAP by using their ETH email address (Resolves Login with email address #598)

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (0bb8db1) 94.77% compared to head (1a52cd1) 94.76%.

Files	Patch %	Lines
amivapi/auth/sessions.py	57.14%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #611      +/-   ##
==========================================
- Coverage   94.77%   94.76%   -0.02%     
==========================================
  Files          75       75              
  Lines        4310     4315       +5     
==========================================
+ Hits         4085     4089       +4     
- Misses        225      226       +1     

Flag	Coverage Δ
unittests	94.76% <66.66%> (-0.02%)	⬇️

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[suisseWalter]

lgtm, doesn't change the behaviour for extraordinary members hat sign in with their private email, but allows eth students to sign in with their full email address. which is now common. also fixes the the issue with signins for extraordinary members, that haven't changed their email/password.