Clarify update vs new donation UX

[jonathanstegall]

For at least some users, it's not clear when they go to update their donation if they are updating an existing one, or creating a new one.

We should figure out if we need to make variations for updating payment method vs updating amount, or if there are other UX improvements we can make.

[jonathanstegall]

In a user's account, it looks like this:

But in reality, "update payment method" and "change amount" are literally the same destination. Maybe they should be separate forms.