Updates to donate page

donate: make email less trivial to crawl

We've been seeing some spam coming into this mail address. Add some
simple HTML tags around parts of the mail address that are not printed
in the browser and shouldn't even be annoying for screen readers. I've
done something similar with my mail address on my blog and not been
receiving much spam. Of course this doesn't help with the cases where
the address has already been crawled, but if we don't do this we may
receive even more in the future.

donate: update text regarding small donations

I've talked to NLnet recently, and it became clear that having many
small donations takes a lot of effort to process. Adjust the wording.