Montlhy donations refactor

Here's a Lucid chart:
https://www.lucidchart.com/documents/edit/b89340cd-51c4-4dd7-933f-76f4e61863d8/0

The old recurring payments / direct debit structure is taken down. It used to be spread over donations and fund app.
New monthly donations are run in two stages. First step is to prepares a MonthlyBatch with objects similar to actual Orders and Donations. These can be reviewed and then processed.
For now the two steps are run through management commands:

./manage.py process_monthly_donations --prepare
./manage.py process_monthly_donations --process

For inconvenience there's  also a command to run a single prepared donation:
./manage.py process_monthly_donations --process-single some@email.com

There are checks so that you can't create a new batch if there's one younger then 10 days. Likewise when processing the orders it won't create a new (actual) Order if there's an Order with recurring=True for that user within the last 10 days.

There are some API tests for setup up a monthly donation. What's still missing is a bunch of test for the management commands. Maybe we could postpone those?