Make bottom margin in field template configurable

[medihack]

It would be nice to have the bottom margin mb-3 of the field template configurable. Currently, it is hard coded there. Also discussed some time ago in the Django forums.

[mconigliaro]

More discussion: #128

[dwasyl]

This would be very much appreciated as a change. I just went down a whole rabbit hole to figure it out and landed here.