Initialise dialogOpen value to false

Because it was undefined, the toggleClass method just turned the class on
if it wasn't there in the first place.

Silly jQuery, Trix are for kids.

Change-Id: I4e21e033529834a4f808270306c72f5756b69ab2
Mingle: https://wikimedia.mingle.thoughtworks.com/projects/multimedia/cards/921