Add responsiveness for footer. Update gulpjs, package.json and bootstrap sass

static/assets/bootstrap/sass/bootstrap-grid.sass

@@ -0,0 +1,31 @@
+/*!
+ * Bootstrap Grid v4.1.2 (https://getbootstrap.com/)
+ * Copyright 2011-2018 The Bootstrap Authors
+ * Copyright 2011-2018 Twitter, Inc.
+ * Licensed under MIT (https://github.com/twbs/bootstrap/blob/master/LICENSE)
+
+@at-root
+  @-ms-viewport
+    width: device-width
+
+  // stylelint-disable-line at-rule-no-vendor-prefix
+
+html
+  box-sizing: border-box
+  -ms-overflow-style: scrollbar
+
+*,
+*::before,
+*::after
+  box-sizing: inherit
+
+@import functions
+@import variables
+
+@import mixins/breakpoints
+@import mixins/grid-framework
+@import mixins/grid
+
+@import grid
+@import utilities/display
+@import utilities/flex