make templates compatible with django-template-preprocessor

this patch add the support to django-templates_preprocessor
compiler to remove useless spaces in output of bootstrap forms
i just replaced the line
```
{% for attr,value in field.field.widget.attrs.items %}{{ attr }}="{{
value }}" {% endfor %}
```
by a {{ field.field.widget.attrs|display_attr}} with the newly created
tag `display_attr`