Make show_roles language independent

A menu in the membership form was displayed depending on whether
show_roles was equal to "No", however as the initial boolean value was
passed through the Django yesno template filter which is
internationalised, this would not work when using the dashboard in
other languages.

Change-Id: Ie58eb98590296b0c9f38b3c243c7c63327755fd9
Closes-Bug: #1224338
Merge "Make show_roles language independent"