FIX: recurring events not displayed on /upcoming-events

[renato]

When we introduced the DiscoursePostEventEvent model and the API service, the recurrent events stopped working as their logic still relied on the old naming convention, upcoming_dates.

This PR changes the addRecurrentEvents logic to work with and to create DiscoursePostEventEvents for each of the upcomingDates, and changes the logic on the Category calendar to also work with the DiscoursePostEventEvent instance instead of the plain object, as well as re-using the API service to fetch the events.

The function passed to onPageChange was also changed to async/await with some minor changes for clarity.