Dispense with static copy, non SASS Bootstrap variant

Using separate submodule repo for Bootstrap SASS allows better
customerization and `@extends` for visual adjustments to Kit.

Author: jordanmccullough

.gitmodules

@@ -16,3 +16,6 @@
 [submodule "_javascript/jquery-scrollsnap-plugin"]
 	path = _javascript/jquery-scrollsnap-plugin
 	url = https://github.com/benoitpointet/jquery-scrollsnap-plugin.git
+[submodule "dependencies/bootstrap-sass"]
+	path = dependencies/bootstrap-sass
+	url = https://github.com/twbs/bootstrap-sass