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

[DEFAULT]
xmpp_domain = server1.test
email_server = sat.test
email_port = 8025
email_admins_list = admin@server1.test

[component file_sharing]
tls_certificate = /usr/share/sat/certificates/server1.test.pem
tls_private_key = /usr/share/sat/certificates/server1.test-key.pem
http_upload_public_facing_url = https://sat.test:8888