[service] enforce that Service argument and return are json-serialisable
The call_service API need to be able to run through RPC. So we ensure front
start that it is possible serialise both input and output.
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.