[web] Add datadir-url configuration option (closes #5204550)
Makes it possible to use a different host for static data instead of ${base_url}/data
Schema of the data
------------------
First take a look at the data schema_ then try to remember that you are browsing
through a heap of data by applying stylesheets to the results of queries. This
site is not a content management system with items placed in folders. It is an
interface to a database which applies a view to retreived data.
.. _schema: ../schema