Add the skeleton of an upload page.

For now, it does nothing, but as usual there are things that do not
work¹:
  - MaxUploadSizeExceededException is not correctly handled;
  - The error message when you are not logged in does not appear for
    some reason, but you correctly get redirected.

And the style of <input type="file"/> and <select/> do not match the
style of <input type="submit"/>.

1: I love the way Java and Spring manage to make everything hard to do
even when it actually seems simple.