RFC: Explicit "donor" status/record for users who donate, to identify among /differentiate from contributors

# Problem

We do not currently have a way to easily differentiate donors among all project/organization contributors. We can technically do so by inferring through subscriptions, but it's not straightforward.

The way this works has not been fully decided yet.

Is a donor status something that's "on" while a donation is active, or if a donation has ever been active? I'll assume the second choice, considering the option of 1-off donations in the future.

My suggested implementation would be

## MVP

- **4 hours** we add a `ProjectDonation` record, associated to `Project` and `User`.
- **2 hours** when a donation occurs, this record is found or created for the user donating and the project being donated to

## Optional

- **? hours __not MVP__** the record could potentially hold additional information such as `total_recurring_amount` and `total_non_recurring_amount`, which could be useful
- **2 hours** we can switch our total donations code to use the above record, once the extra fields are added
- **1 hour** when a subscription get's cancelled (we are not handling this properly yet), we simply nullify the recurring donations field, instead of deleting the record