Made the start date a naive when being sent to the admin overview page.

Luko van der Maas
2017-12-27T23:17:52+01:00

~bug

Closes #516

#### Previous behaviour
Steps to reproduce:

1. Create an event with a start time after now.
2. Look at the event in /admin/events/event/ and compare the start time with the time in the details page or the normal detail event page.
3. compare the registration start with the time on the details page.
4. These are different.

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

1. Create an event with a start time after now.
2. Look at the event in /admin/events/event/ and compare the start time with the time in the details page or the normal detail event page.
3. compare the registration start with the time on the details page.
4. These are now the same.