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

static/assets/bootstrap/sass/utilities/_embed.sass

@@ -0,0 +1,41 @@
+// Credit: Nicolas Gallagher and SUIT CSS.
+
+.embed-responsive
+  position: relative
+  display: block
+  width: 100%
+  padding: 0
+  overflow: hidden
+
+  &::before
+    display: block
+    content: ""
+
+  .embed-responsive-item,
+  iframe,
+  embed,
+  object,
+  video
+    position: absolute
+    top: 0
+    bottom: 0
+    left: 0
+    width: 100%
+    height: 100%
+    border: 0
+
+.embed-responsive-21by9
+  &::before
+    padding-top: percentage(9 / 21)
+
+.embed-responsive-16by9
+  &::before
+    padding-top: percentage(9 / 16)
+
+.embed-responsive-4by3
+  &::before
+    padding-top: percentage(3 / 4)
+
+.embed-responsive-1by1
+  &::before
+    padding-top: percentage(1 / 1)