Allow organisation members to join

[russss]

Hackspaces should be able to join as members. We should probably ask the user applying to sign up for an account first (this won't be an individual member but it will be the first user associated to the space).

Info we need for the space is:

• Display name of space
• URL
• Organisation name and address
• Yearly payment amount (we should allow spaces to specify their own payment amount, but make the recommended/expected amount visible on the page). Minimum £20/year.
• Gocardless direct debit mandate setup

Then we'll need an admin method to approve spaces (could just be Django admin for the moment). Once the space is approved as a member then we can schedule a direct debit payment as mentioned in #5.

[Axford]

Notes on gocardless implementation: https://github.com/UKHackspaceFoundation/website/wiki/GoCardless-implementation