[CWEP002] introduce RQLRelationRewriter Refactor existing RQLRewriter for later reuse of rewriting relation as specified by CWEP002. Work is different because we simply want to replace a relation by another rql snippet and we don't have to bother with EXISTS, subqueries and all. This rewriter is not yet plugged into the querier. Depends on yams 0.40 API. Related to #3546717

Add some entities
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As manager, you can access to entity creation forms by using the `site management`_ page.

.. _`site management`: manage

As regular user, the index page or the action box may propose some links to create entities according to the context.