New-user role default should be "Contributor," not "Super"

[dsalo]

In the role-assignment dropdown in the Add New User screen, the default role is "Super." The default should instead be the lowest-privilege role, "Contributor."

The "Super" default is problematic for security because it inclines Omeka admins to violate the Principle of Least Privilege by accidentally/thoughtlessly/cluelessly assigning unnecessary system privileges to new users. It is also a tiny bit annoying for admins because typically, most users on an Omeka instance will be Contributors, not Supers.

(Since this touches on security, I did use the security email first, and was encouraged to post an issue.)