use a proper authentication flow

At the moment, we have 2 secret files:
.env for LDAP related secrets
credentials.json for matrix related secrets

Most matrix servers allow for password-login, so we can just grab the access_token with the normal login and update it automatically if needed (or the user account changes).
TUWien is special in this regard, since they dont allow for a password login and therefor we need to define the access_token by hand