Reset add block dropdown after adding one
Reset add block dropdown after adding one

It's possible to close a popup without the HTML of a column changing - so the dropdown needs resetting incase a user wants to add the same block again

Fixes #79

developersociety/django-glitter,blancltd/django-glitter,developersociety/django-glitter,developersociety/django-glitter,blancltd/django-glitter,blancltd/django-glitter