Summary: Allow organizations to add their own options to the payment method dropdown.

What: Let admins create payment method labels, such as Stripe, when recording payments received outside Givebutter.

Why: Organizations want the payment source recorded in the payment method field instead of choosing Other and adding an internal note.

Customer context: A customer saw Venmo and PayPal options and asked whether they could add Stripe themselves. They already know about the internal-note workaround.