Allow board membership to contain end date

Ghost User
2019-03-27T19:50:29+01:00

Closes #416

#### Previous behaviour
Steps to reproduce:

1. Enter end date for Board membership; this produces an error message.

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

1. Enter end date for Board membership; this no long...

Closes #416 (closed)

Previous behaviour

Steps to reproduce:

1. Enter end date for Board membership; this produces an error message.

New behaviour

Steps to validate that it works:

1. Enter end date for Board membership; this no longer produces an error message.