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

static/assets/bootstrap/sass/mixins/_image.sass

@@ -0,0 +1,32 @@
+// Image Mixins
+// - Responsive image
+// - Retina image
+
+// Responsive image
+//
+// Keep images from scaling beyond the width of their parents.
+
+=img-fluid
+  // Part 1: Set a maximum relative to the parent
+  max-width: 100%
+
+  // Part 2: Override the height to auto, otherwise images will be stretched
+  // when setting a width and height attribute on the img element.
+  height: auto
+
+// Retina image
+//
+// Short retina mixin for setting background-image and -size.
+
+// stylelint-disable indentation, media-query-list-comma-newline-after
+=img-retina($file-1x, $file-2x, $width-1x, $height-1x)
+  background-image: url($file-1x)
+
+  // Autoprefixer takes care of adding -webkit-min-device-pixel-ratio and -o-min-device-pixel-ratio,
+  // but doesn't convert dppx=>dpi.
+  // There's no such thing as unprefixed min-device-pixel-ratio since it's nonstandard.
+  // Compatibility info: https://caniuse.com/#feat=css-media-resolution
+  @media only screen and (min-resolution: 192dpi), only screen and (min-resolution: 2dppx)
+    // Standardized
+    background-image: url($file-2x)
+    background-size: $width-1x $height-1x