Feature/fix date time

[BerendWouters]

Multiple issues are fixed in this PR:

• For some reason the JSON serializer in Angular doesn't take moment objects properly into account. This has been fixed by using .format() on the moment object. It's still dirty, but it does the trick
• Updating events didn't update the event slots, causing the start and end date not to be updated anyway. The current implementation takes some shortcuts, but will recreate eventslots now.