Restore site war dependency on webfiles module for code completion

ARCHE-457 added a dependency to the pom of the webfiles module, unfortunately without mentioning in the issue why that was done (This is ARCHE-456). It was removed by issue ARCHE-527 as 'meaningless' which is unfortunately not true.

This dependency makes code completion possible for Beans in the freemarker templates.

Typical comment in ftls:

<#-- @ftlvariable name="document" type="org.example.beans.NewsDocument" -->

This lets intellij give you code completion on ${document.XXXX} code. But it needs that dependency to the site war for it to work.

Please restore this to make development work easier.