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

static/assets/bootstrap/sass/utilities/_background.sass

@@ -0,0 +1,14 @@
+// stylelint-disable declaration-no-important
+
+@each $color, $value in $theme-colors
+  +bg-variant(".bg-#{$color}", $value)
+
+@if $enable-gradients
+  @each $color, $value in $theme-colors
+    +bg-gradient-variant(".bg-gradient-#{$color}", $value)
+
+.bg-white
+  background-color: $white !important
+
+.bg-transparent
+  background-color: transparent !important