Silva-core-services

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* Interfaces for Members and Groups services have been added/moved here.

* A new default service is added: Silva Secret Service, that create a
secret out of some strings and validate it.

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* Add an utility ``walk_silva_tree`` to go through all Silva content.

* Simplify catalog setup code with new ``silva.core.conf`` default
service factory.

* Remove ``haunted_path`` index which now useless.

* Update code to be compliant with Zope 2.12.

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* Initial release.