Added full Github and Google+ URLs to student tables

The person in charge of iLabs wants full URLs to show up in the student tables
(students/index and students/past_students) so that he can see what students
entered for their URLs more quickly.
Allow instructors to edit StudentPreferences

The person in charge of UNL iLabs wants to be able to edit any of a student's
profile settings. A lot of them are tied to the StudentPreferences class, so
this commit gives admins the ability to add and edit those StudentPreferences.