Make "default" authentication mode pre-selected in "Login" dialog configurable

**Problem:**
This is currently not configurable per-se. It is currently tied (hard) to the configuration option dmx.signup.ldap_account_creation. Meaning, if the sign-up form is configured to create LDAP accounts, "LDAP" authentication mode is pre-selected as default (in the sign-up login dialog).

**Solution:**
I don't know. To be discussed an defined.