Collective dues

API PR here: membership_api!1 (closed)

This PR implements collective dues for Silicon Valley DSA, as approved by vote during the January branch meetings.

Collective dues are a one-payment scheme where members pay membership dues directly to the chapter and specify how the dues are to be split. The chapter then portions out the dues intended for National and pays for all covered memberships at once. If new members cannot pay dues, the chapter sponsors and subsidizes their dues instead of waiting for a dues waiver from National (which in SV DSA's experience can take multiple months to process).

This PR (and the corresponding API PR, pending) implement this scheme using Stripe Billing and a subscription-based model. Each enrolling member subscribes to two Stripe products: National dues and Local dues. Each product is priced according to the amount the member is able to pay (according to a Pay-what-you're-able model).

The scheme comprises a part of a "Join DSA" form, pictured here:

Member is able to pay

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Member requests sponsorship

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See home#1 (closed) (access for DSA members)

Associated DSA SF PR here: DSASanFrancisco/membership_ui!361 (merged)