plugin invitation: pubsub invitations: - new Pubsub invitation plugin, to have a generic way to manage invitation on Pubsub based features - `invitePreflight` and `onInvitationPreflight` method can be implemented to customise invitation for a namespace - refactored events invitations to use the new plugin - a Pubsub invitation can now be for a whole node instead of a specific item - if invitation is for a node, a namespace can be specified to indicate what this node is about. It is then added in `extra` data - an element (domish.Element) can be added in `extra` data, it will then be added in the invitation - some code modernisation

You'll find install instructions in documentation (see doc/installation.rst).

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