refactor: Swap crypto QR modal header from H4 to H3 to avoid double underline

When using one's own wallet app to donate cryptocurrency to Tor, if using a Javascript-enabled browser and if using the QR code to obtain the relevant Tor wallet address, a modal will pop up over the site to center the QR code with a simple title denoting its particular (crypto)currency.

Prior implementations of this modal suffered because it used h4 for the title, which wound up (due to CSS rules inheritance) adding an extra dividing line between the title and content - because the default Bootstrap modal layout brings its own.

This commit belatedly adjusts the relevant SASS in static/sass/components/modal.scss to remove the extra border (and space that would have added distance between that border and the following content - as, again, the modal brings its own). This provides a much cleaner modal, in all.