Ldap import on initial login

This PR does the following:

* Upon a successful login of an unknown user, his or her data is fetched from LDAP using their credentials.
* If someone successfully logs in, and no LDAP synchronization has taken place in the past 14 days, their LDAP credentials are used to pull every student and employee from the LDAP server and store them in the database. Existing users remain unmodified in the database, whilst missing users are removed from the database.