Clarify and improve the platform fees

[chrislo]

The current description of the platform fee is:

To cover the costs of running and building jam.coop a 15% fee (minimum 20p) is taken from each sale.

I'd like to extend this to encourage larger artists and labels to take a punt on jam by offering lower platform fees based on volume of transaction. I'm thinking

• 15% base platform fee
• 10% for artists who have lifetime sales >£1000
• 7% for artists who have lifetime sales >£5000

Additionally I'm thinking we could focus on the third tier of artists for the initial membership/profit-sharing offer. If we get that far and have some committed/economically engaged artists/labels to work with, it'll give us an opportunity to collectively decide what membership looks like before extending it to other artists.