Replace query parameter by anchor in dynamicaly generated pages

[benoit74]

See #106 (comment)

In all dynamically generated pages like "unavailable_offline", we use query strings to indicate dynamic values that has to be used in the page.

This is now known to me to be a bad idea. It only works due to kiwix-serve dropping the query parameter before transferring the request to libzim. This is known to have issues in other readers.

The queryparameter must be replaced by a magic anchor.