[Refactor] Clean up donation creation flow

[dimitur2204]

Which area(s) of Podkrepi.bg are affected? (leave empty if unsure)

No response

Describe the feature you'd like to request

After recently having problems with the Stripe integration it seems like we have a lot of places to improve on the flow of data when creating a donation.

Related issues:
#428
#463

Describe the solution you'd like

Think about a better more reusable and understandable approach on handling donations. This can come together with podkrepi-bg/frontend#1324, so that the whole flow both frontend and backend gets refactored.

Describe alternatives you've considered

No response