fixes #3827 - ldap avatar support

I know this is one of the most inconsequential things ever.  It pulls the jpegPhoto attribute from LDAP as a user avatar.

~~Currently store it in the DB... (terrible, I know).  My avatar was absolutely tiny at only 4kb.  I could write to public/avatars/<hash>.jpg too, that's probably more sensible?~~

Avatars are written to /public/avatars/[hash of image].jpg.  Changes are captured if the data in LDAP changes -- I delete the old avatar and add the new one.

This capturing of LDAP changes has the added benefit of also capturing if the other attributes are changed (first name, last name, mail, etc) for a user.