Saving events with same title fails

[jeverling]

Hi, and many thanks for this cool project! I want to use it as a workout tracker, so I'll have repeating events that are not recurrent events, e.g. Deadlifts.
Wagtail refuses to save EventPages with the same slug. It's not a big deal to update the slug manually, but it would be nice if jouyous would take care of that automatically. I can imagine several use cases where it can happen over time that someone tries to create a new event with a title that already has been used at some point before.
I have created a small WIP PR with an approach based on https://stackoverflow.com/a/43803611

If you think that's a good idea, I'm happy to add tests, or maybe even a settings flag if you think it would be better to make this optional.