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

static/assets/bootstrap/sass/mixins/_screen-reader.sass

@@ -0,0 +1,30 @@
+// Only display content to screen readers
+//
+// See: https://a11yproject.com/posts/how-to-hide-content/
+// See: https://hugogiraudel.com/2016/10/13/css-hide-and-seek/
+
+=sr-only
+  position: absolute
+  width: 1px
+  height: 1px
+  padding: 0
+  overflow: hidden
+  clip: rect(0, 0, 0, 0)
+  white-space: nowrap
+  border: 0
+
+// Use in conjunction with .sr-only to only display content when it's focused.
+//
+// Useful for "Skip to main content" links; see https://www.w3.org/TR/2013/NOTE-WCAG20-TECHS-20130905/G1
+//
+// Credit: HTML5 Boilerplate
+
+=sr-only-focusable
+  &:active,
+  &:focus
+    position: static
+    width: auto
+    height: auto
+    overflow: visible
+    clip: auto
+    white-space: normal