Use relative URL to call the encryption script

To enable running this script from URLs other than the server root, use
relative URLs.
js: Get rid of some jquery code

Convert jquery calls to normal JS/HTML when reasonably possible.
The only remaining jquery dependency is the UI CSS sheet which is used
for by the <button> elements in my HTML.
Convert <button> to <div>

To get rid of the last jquery parts, the <button> elements are converted
to regular <div> elements + CSS magic (more or less copied from the
jquery styling).