Upgrade xstatic-bootstrap-scss to at least version 3

Horizon did the jump and upgraded to Bootstrap 3 finally, so now
we need the corresponding version of packaged bootstrap for it.

Change-Id: Ic76c7e8b4c5a89b724d1792e9538849da3ab8dd4
Implements: blueprint remove-javascript-bundling
Merge "Upgrade xstatic-bootstrap-scss to at least version 3"