Have non-admin user's in Plinth

Created by: SunilMohanAdapa

We should have users in Plinth who are non-admin users. Currently each user in Plinth is an administrator. If the FreedomBox owner wishes to provide friends and family with accounts these need to be non-admin users. Imagine a user logging into Plinth to download OpenVPN profile but not being able to alter anything else.

Various functions in FreedomBox need to check for admin privileges.