Do not show cohort 2019 yet on the members overview page

Ghost User
2019-02-13T19:06:33+01:00

Closes #824 ISSUE

#### Previous behaviour
Steps to reproduce:

1. Go to the members overview
2. Already shows filter for 2019

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

1. Go to the members overview
2. No filter for 2019 yet, after 1 september there will be