Improve /events API

[daxadal]

Base tasks:

• [x] Rename eventsApp as router
• [x] #25
• [x] Rename error messages from error to message
• [x] Use enum for states
• [x] Trying to modify or delete other user's event should return 403, not 400
• [x] Limit of concurrent subscriptions extracted to constant
• [x] #26
• [x] Prefer body desestructuring