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

static/assets/bootstrap/sass/mixins/_background-variant.sass

@@ -0,0 +1,16 @@
+// stylelint-disable declaration-no-important
+
+// Contextual backgrounds
+
+=bg-variant($parent, $color)
+  #{$parent}
+    background-color: $color !important
+
+  a#{$parent},
+  button#{$parent}
+    +hover-focus
+      background-color: darken($color, 10%) !important
+
+=bg-gradient-variant($parent, $color)
+  #{$parent}
+    background: $color linear-gradient(180deg, mix($body-bg, $color, 15%), $color) repeat-x !important