This gives us the fixes versions of cgi and multipart, as well as aeson
2 (which required some code adjustments).

This way we avoid the right-drift that we introduced in the first
version, as the error handling and propagation is done in the Maybe
monad.

The downside to this is that we lose some error information: We cannot
show anymore if the authorization failed or something else went wrong,
because in all cases we just bail out with Nothing.

This switches some definitions around and wraps App in a newtype, in
preparation for more cleanup.

The right-drift was not nice to look at, so we're now doing the dispatch
using a small helper method.

The original landing page was pretty bad, not just from the looks, but
also from the functionality: In order to paste an image, you had to have
the cursor focussed in the small div.

This edition brings several improvements:

* The general style of the page has been improved by adding some CSS
  styling to adjust the colors and center the content.

* Pasting an image will not work everywhere on the page without
  focussing a specific element. This seems to work because
  document.onpaste fires every time, not just when the element is
  focussed and it even works without contenteditable=true, which removes
  the ugly blinking cursors.

* There is a preview window for pasted images, clearly indicating what
  is going on and which image is going to be pasted.

* Pasting an image will now hide the file chooser.