Dashboard: Update mimetype to content_type in HttpResponse

refs #7305

mimetype is deprecated in Django 1.7, replaced by content_type
ATK: Update Form to take advantage of ModelForm

Use ModelForm's ability to autogenerate fields from the Model, instead
of redefining fields. Move labels and choices to the model.
Forms: Add explicit fields to all ModelForms

refs #7305

Django 1.7 no longer defaults to fields being everything because that
is a security risk. Add explicit specification of fields to all
ModelForms.