Unbundle bootstrap and use xstatic version

This uses the xstatic-packaged versions of bootstrap and bootstrap-datepicker.
It requires newer versions of pyscss and django-pyscss, as there were some
bugs in there that were just fixed upstream.

Change-Id: I0a7660d29fecde915eb46916453a9ea96296798d
Implements: bluperint remove-javascript-bundling
Closes-bug: #1375883
Merge "Unbundle bootstrap and use xstatic version"