REPLACE endpoint for more reliably replacing events

So it's already possible to replace events by setting the "id" property in the /insert endpoint.

So why would you want to have a separate /replace endpoint?
- Because /insert does not fail if the "id" does not exist, it creates a new event
- If one out of many replaces in /insert fails we have no idea what actually happened, we need to do fetch the events after to be certain which should be unnecessary
- Any other reasons, put a comment below and I'll add it to this list

I don't think this is a high priority task right now, might be in the future if we get a use-case which replaces a lot of events.

Resources:
- Discussion on aw-server-python PR by @leoschwarz https://github.com/ActivityWatch/aw-server/pull/57