Some things are not clear to me with LDAP.

1. If a user is to be authenticated by LDAP - does she need pre-creation of sqlpad user by admin? This should be documented in authentication.md

2. Is it possible to enable LDAP authentication while disabling local username registration? (It should be possible in an enterprise environment. Looking at the code I'm not sure this is possible). This should be documented in authentication.md

3. Add a URL example LDAP_URL to configuration.md - I took the example from http://www.passportjs.org/packages/passport-ldapauth/ . Is that correct? are both LDAP and LDAPS supported? preliminary LDAP docs #707

@dengc367