Specify Payment Processor fees when adding funds

[Betree]

From @jfdh:

When adding fund manually to a collective, I'd like to be able to add payment processor fee. As we are dealing with many transactions coming in EUR or USD in our bank account, we must often make currency exchange in our bank accounts, and the bank always charges fees to us for this. Moreover, some foundations in the US are sending USD to our EUR account, which also generates additional banking fee.

@BenJam also mentioned that OSC had a similar need for cases when adding funds from a large donor, who's then annoyed that the reflected amount on the platform does not match what they initially contributed.