Allow events to be favorited (added to dashboard)

[pferreir]

Right now, only categories can be added. Adding events can be pretty useful e.g. for conferences or lectures.

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Main tasks to do here:

• Add UI for users to toggle favorite event state (ideally a small react component)
• Create backend to store the user's favorite events (similar to category favorites)
• Take favorite events into account when determining category suggestions
• Show favorite events on dashboard in "Your events at hand" with a new role icon ("Favorite" ⭐)

[ThiefMaster]

When we add this, we should also take into account that the CERN calendaring plugin should add such events to the user's calendar.