Fix start/end date parsing in events API

Ghost User
2019-08-13T11:43:26+02:00

This is already deployed in a hotfix on v25.

#### Previous behaviour
Steps to reproduce:

1. Visit `/api/v1/events/`
2. Crash

#### New behaviour
Steps to validate that it works:

1. Visit `/api/v1/events/`
2. No crash