Create form to generate Monthly donation reports

We’ve been creating monthly donation reports manually for some time now, and it would be helpful to build an admin form that allows designated staff to automatically generate these reports.

As a first step, we should enable staff to generate the existing monthly donation reports:

• Monthly Paypal donations
• Monthly Stripe donations

Once that’s in place, we can expand the functionality to include additional reports that staff can generate.

/cc @carmendelgado