[multi-sources-removal] drop repository entities_modified_since public api
actually only used by pyrorql/zmqrql sources which have been dropped.
This will allow to drop a bunch of system meta-data (eg deleted_entities)
Related to #2919300
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.