Board membership cannot have end dates

One-sentence description

Board members are currently not allowed to have an end date for their group membership even though we might want them to have one. Examples are the boards of 2012-2013 and 2017-2018.

Current behaviour

Try to enter an end date for a board membership.
Does not work

Expected behaviour

Should work

Edited Oct 14, 2018 by Sébastiaan Versteeg

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