Account for Global Admin, Network Admin, and Site Admin roles

[jeremyfelt]

These will start to differ from the traditional super admin provided for network administration by default.

Roles:

• Global Admin has all access to _the platform_ and can manage all networks and sites.
• Network Admin has all access to _individual networks_ and can manage all sites under that network. This role can be applied on several networks for a user.
• Site Admin has all access to _individual sites_ and can manage content and users at that level. This role can be applied on several sites for a user.

We should account for:

• A user can be a Site Admin on one site and a Network Admin on a network that does not encompass that site.
• A workflow for creating new networks.
  • This may be something a trusted Network Admin can do if they already manage multiple networks for a college or department.
  • This may also be something for a Global Admin or helper role with additional capabilities at the global level.

[jeremyfelt]

The concept of these roles was introduced in #103. More work will continue to shape the capabilities of these roles.