Create initial member models

[tyndyll]

Create a new app called membership and create two initial models

• Member - representation of a member, using the Django user model as a ForeignKey (see Django Docs for more info)
• Group - a group within the organisation. These are not used for permissions, but rather for members to self select themselves for e.g. communications send from that group

NOTE: for the member model the on_delete option should be set to PROTECT to keep track of previous transactions

Definition of Done

• Models and migrations created
• str function and test created for models
• Models added to admin.py
• App added to settings.py