Add feature to send a new user a copy of their membership agreement and their payment plan agreement

[jabelone]

If enabled, a user should receive on signup, and be able to request a copy of the membership agreement and the billing agreement that they are currently signed up to. The portal does not support file uploads/persistent storage, so an external link to a PDF would be best for this. For auditing purposes, there should also be an option to email the admin contact every time someone completes a signup.

Current plan:

• [ ] add a feature flag for enabling the membership agreement feature
• [ ] add a config option for a link to the membership agreement
• [ ] add a step during the signup process (perhaps as part of the payment confirmation screen) where the user agrees to the membership agreement
• [ ] add a feature flag to enable automatically sending an email with a copy of the membership agreement and the billing agreement to the member/admin contact
• [ ] add an option to request an email copy of these two agreements to the member's profile page

[ergodone]

As a wish-list item but slightly relevant in this context, we'd like to see support for legally binding contracts via an e-signature. I added a feature request for this #260