Use django-bootstrap3 package instead of local bootstrap

Just like we did in DMT, this uses the django-bootstrap3 package instead of a local copy.

If you'd rather not rely on an external CDN, maybe we could have an internal CDN kind of like we do for pypi, and then configuring our django-bootstrap3 to point to it.

- Also update `django-bootstrap-form` to 3.1.0 which seems to be working fine in DMT.