[migration] move 'entities' table changes from 3.19.0 to bootstrap script
The 'mtime' and 'source' columns need to go away before we attempt to do
anything else with the repo, otherwise any addition of an entity is
going to explode.
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.