Fix column_width filter in python3

Force integer division otherwise we'll fsck bootstrap classes

As seen here:
https://gist.github.com/ScreenDriver/86a812b7b3f891fe8649#file-broken_fieldsets
test_django_admin_bootstrapped: put more than one field in one line

So we can see how crap they look like

See #110
django-admin-bootstrapped: fixup fieldset column with calculation

When we have rows with different numbers of field they look
randomly positioned. Fix that by using the same column width as
the row with more fields.

Fixes #110