LDAP Authentication

"Update" to [Trac #11501](http://trac.sagemath.org/sage_trac/ticket/11501)

Optional user authentication via LDAP (only visible in the notebook settings if `python-ldap` is installed)


(in order to not list all users found in LDAP, the "share worksheet" page now has a search box instead of the list. [Bonus: search results are added to the textbox if you click them])