add collapse.js if we have collapsable inlines

[kviktor]

trafficstars

Earliest Django version with it I could find is 1.10.x: https://github.com/django/django/blob/stable/1.10.x/django/contrib/admin/options.py#L1860-L1861 but https://github.com/django/django/blob/stable/1.9.x/django/contrib/admin/helpers.py#L76 also exists so I think it should work under 1.9.x but I only tested on 3.2.x.