Make donation buttons more obvious

[zyphlar]

Right now the icons at https://github.com/organicmaps/organicmaps.github.io/blob/master/templates/shortcodes/donate_buttons.html look like a bunch of logos and it's not immediately obvious that they're the primary clickable method of donating. At the very least, styling each one to look like a button would be an improvement. Ideally some language would be inserted before and among the buttons to make it just as clear as the bank transfer methods what each logo means.