[etwist] clear ui cache at server startup time. Closes #1806933
this avoid pb when cubicweb/cubes provide new version of some js/css and also accumulating junk in the cache directories
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.