Updated notification - already donating checkbox

### Describe the feature you'd like to request

Every time Floccus is updated and shows the donation options, I go "I think I am already donating, but I'm not 100% sure", so I waste some time checking the various providers until I find my donation.
It would be nice if I could avoid that.

### Describe the solution you'd like

A simple a checkbox with "I'm already donating", which keeps state between updates would do the trick.
No need to hide the donation buttons or anything, I don't feel like Floccus is annoyingly nagging me.

### Describe alternatives you've considered

I've tried remembering better, but so far it's not working.