[webrequest] introduce an alternative implementation using the repoapi DBAPI specific parts of ``CubicWebRequestBase`` are extracted in a ``DBAPICubicWebRequestBase`` decicated class and a new ``ConnectionCubicWebRequestBase`` is introduced that provide the same services but using a repoapi.ClientConnection as data source. For now the ``DBAPICubicWebRequestBase`` is still used by default.

Add some entities
-----------------
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.