[INCOMPLETE] Add partial payments.

Successor to #1947 after I sent the PR to master instead of develop, bringing tons of unwanted commits along.

 - [x] Add partial payments to schema.
 - [ ] Update prices shown to reflect remaining price.
 - [ ] Use partial payments when actually purchasing.
 - [ ] Add UI/API for paying part of a reward.
 - [ ] Decide on whether and how death affects savings.