User flow to batch contributions under a single payment method

[Betree]

User story

This is an alternative proposal to #1916, please see the original issue for the motivation. I think it was initially proposed by @znarf or @alanna orally, I was not able to find an existing issue.

The core of this proposal is to have a flow for users who make multiple contributions to the same host to group them in a single batch. This would have the following advantages over #1916:

• This would work regardless of the payment method (eg. it would be simple to support PayPal)
• We wouldn't need to force contributions to be on the 1st of the month: just let users pick their start date when batching
• We would make sure that users update their payment methods to use the same everywhere, making batching effective

Best solution for this problem

Create batch

• Users pick the contributions they want to batch. Ideally, they would be able to add new ones on the flight
• Users can set a global platform tip
• (PayPal only) Users can pick the date of their contribution
• When users confirm, the system creates single subscriptions for all contributions

Edit batch

We would do something for editing recurring contributions. Maybe instead of the collective card, we would need to link to a separate page to edit the bundle.

Incentive users to create subscriptions

Because it needs manual intervention, the success of such a feature would depend on our ability to surface it nicely. We could:

• Email existing contributors: You could save $1/month in fees to Babel and Webpack, $12/year! Click here
• In the contribution flow, offer to batch the contribution with any existing one if that's possible

MVP

TBD

Documentation

See #1916

[piamancini]

app.fundoss.org has a good model for this, it puts contributions on a card and you pay at the end of it - it can be a first step. A user can have multiple 'batches' and then we batch the whole thing eventually.

[darkdragon-001]

I think a first step for implementation could be to create an API as described in #6025 but still executing the payments individually. The next steps could be done in parallel: batching the payments internally and adding UI functionality.