Add "group" app

[brylie]

There exist several types of groups related to sustainable urban development and carbon reduction. Some examples of common terms include a network of cities, a consortium, or an alliance.

Immediate need

We want to document the structure and activities of the Hinku network.

Design

Groups should have the following attributes:

• name
• description
• website

Groups may be hierarchical, so consider using a tree model, such as django-mptt.

Groups can relate to AdministrativeArea(s) (#153) and Project(s).