Ftw.participation

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* Set "came_from" if user was invited.
[mathias.leimgruber]

* Select additional roles for invitations
[mathias.leimgruber]

* Added condition for invalid invitations.
[mathias.leimgruber]

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* Fix in participation.pt - don't show groups
[mathias.leimgruber]

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* load plone.app.workflow
[mathias.leimgruber]

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* Initial implementation
[jbaumann]