Prettier 2: Electric Boogaloo (#6113)

# Description of Changes
When I added Prettier formatting in #6052, my aim was to use just the
default settings in Prettier. Turns out, Prettier looks _really hard_
for any config files if it's not explicitly given one, which means that
if a developer has some sort of Prettier config file lying around on
their system, Prettier might find it and use it. Also, Prettier changes
its defaults based on stuff in `.editorconfig` without any good way of
disabling that behaviour explicitly in its config file.

To solve both of these issues, I've introduced a `.prettierrc` file
which sets Prettier's defaults explicitly, and then reformatted all our
code _again_ in Prettier's actual default settings. This should achieve
the aim of #6052 and remove the possibility for it breaking on different
dev computers.

frontend/src/saas/routes/authShared/GuestSignInButton.tsx

@@ -8,7 +8,11 @@ interface GuestSignInButtonProps {
   disabled?: boolean;
 }
 
-export default function GuestSignInButton({ label, onClick, disabled }: GuestSignInButtonProps) {
+export default function GuestSignInButton({
+  label,
+  onClick,
+  disabled,
+}: GuestSignInButtonProps) {
   return (
     <button
       type="button"