Configure registration validation on server side

As part of #98, \BlueLaTeX server administrator can choose how newly
registered users validate their registration.
Propose password input when registering

Depending on the configuration returned by the server, the client
decides whether to require registering users to input their password in
the registration form.

This is part of resolution of #98
Merge pull request #174 from satabin/feature/configurable-email-validation

Feature/configurable email validation