fix: Spacing b/w major elements adjusted to match comp

A fix for the issue described in this ticket - #31 - which has a (tremendously helpful) image overlaying some screenshots of the donation form with the intended between-element spacing.

While a great deal of these changes amount to CSS tweaks, the spacing between Stripe's embedded form elements was achieved in Javascript, by extending the Elements object's "appearance" setting with the gridRowSpacing variable, as documented here: https://docs.stripe.com/elements/appearance-api#variables

Closes: #31