Improved the Shortlink feature

- Shortlinks now also work without a fragment as a prefix.
  In that case the client is forwarded to the adress with a fragment.
- There now is an error page for Shortlinks, that is displayed when a Shortlink is not found,
  but was entered without a fragment.
- Relates to issues #12 and #8
Now displaying a modal when the page loads something.

- The modal needs translation into other languages, which will become another issue.
- Closes #22