Protect auth endpoints against CSRF login attacks

[OmeGak]

Indico seems to be vulnerable to CSRF login attacks per OWASP Cross Site Request Forgery article:

An attacker can use CSRF to obtain the victim’s private data via a special form of the attack, known as login CSRF. The attacker forces a non-authenticated user to log in to an account the attacker controls.

This PR protects authentication endpoints against CSRF.