Endpoint to delete payments

[mschader]

Currently, it is not possible to delete payments. With the implementation of #13, this should be implemented. This feature should only be available with #13, as stated in OpenSplit/opensplit-frontend#10.

TODO:

• Implement API endpoint for payment deletion
• Update documentation for new API endpoint

[mweinelt]

I think transactions should only be deletable within a grace period of {24,48,96}h, after which they become immutable/frozen.