Rewrite /donate without Bootstrap
As part of #18142, I already removed Bootstrap on /contribute/personas and /contribute/mission because I felt safe doing so, though you can have a look too!
But /donate is more tricky and sensitive.
Fun fact, writing custom CSS code actually removes 20% of the lines of code.
It was also the last page, so I'm removing everything else related to Bootstrap.
I tested several donation scenarios on PayPal, one-time and recurring.