Introduce a Pricing Plan

User Story:

As a User I want to see the various plans available so that I can make a choice which one I want
As a User I want to sign up for a given plan
As a User I want to change my plan

Note: project will remain fully-open source and there will always be a zero cost price plan

Details

Spec out
Pricing page: list the plans, link to sign-up
- Anonymous / signed-up distinction (??)
Integration into user objects - need to store their plan type
Enforcing plan parameters in the app
Payment processing - propose we use stripe
- handling lapsed payment etc
  Why?
Provide income to support ongoing support and maintenance of the project
Users actually expect to see a "pricing" page and it gives them confidence in the status and sustainability of the project
Provide input on the "demand curve"