Add select2 class to choice fields

This makes multiple choice drop down lists in Alert Profiles filter
searchable using select2, which makes it a lot easier to find desired
values when the choice lists become huge.

python/nav/web/alertprofiles/forms.py

@@ -734,6 +734,7 @@ def __init__(self, *args, **kwargs):
 
                 # At last we acctually add the multiple choice field.
                 self.fields['value'] = forms.MultipleChoiceField(choices=choices)
+                self.fields['value'].widget.attrs['class'] = 'select2'
             else:
                 self.fields['value'] = forms.CharField(required=True)