Groups or Role-based authorization

[joepio]

The current Hierarchy #18 model allows for setting read and write rights, but it is very basic and definitely not good enough for more complex usecases.

Must haves:

• Define Roles and define how these grant rights in specific contexts
• Add and remove Roles to users, which in turn grant rights to sets of Resources
• See which Roles a users possesses in some context (Maybe a Drive?)

Nice to haves:

• Set durations for Roles, so they are temporary

[joepio]

Discuss further in #73