Change `ExtraInfo` to user fields, add search
Change `ExtraInfo` to user fields, add search

`Register_cme/extrainfo` in Django Admin was previously displaying users
as `ExtraInfo` objects which admins had to click on individually to see
each user's information. Each user is now displayed with fields:
username, email, last and first name. Username is clickable to view more
information. Added search bar enables search for users matching query
for username, email, last and first name.

Stanford-Online/edx-platform,caesar2164/edx-platform,Stanford-Online/edx-platform,Stanford-Online/edx-platform,Stanford-Online/edx-platform,caesar2164/edx-platform,caesar2164/edx-platform,caesar2164/edx-platform