Debounce meeting participant updates

[oliverguenther]

• Allow participants to be aggregated
• Allow aggregation period to be customized by journal class (we want meetings to be debounced by 1minute)
• Add a debounce job that waits for 1 minute, referencing a journal that we want to aggregate changes since. If new changes come in, that job will be replaced by new one
• Add a notification service DispatchAggregatedNotificationsService that takes these two journals, and then calculates what and to whom we need to send out emails
• Update participants emails so that we can show added/removed in one email