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

static/assets/bootstrap/sass/_close.sass

@@ -0,0 +1,32 @@
+.close
+  float: right
+  font-size: $close-font-size
+  font-weight: $close-font-weight
+  line-height: 1
+  color: $close-color
+  text-shadow: $close-text-shadow
+  opacity: .5
+
+  &:not(:disabled):not(.disabled)
+    +hover-focus
+      color: $close-color
+      text-decoration: none
+      opacity: .75
+
+
+    // Opinionated: add "hand" cursor to non-disabled .close elements
+    cursor: pointer
+
+// Additional properties for button version
+// iOS requires the button element instead of an anchor tag.
+// If you want the anchor version, it requires `href="#"`.
+// See https://developer.mozilla.org/en-US/docs/Web/Events/click#Safari_Mobile
+
+// stylelint-disable property-no-vendor-prefix, selector-no-qualifying-type
+button.close
+  padding: 0
+  background-color: transparent
+  border: 0
+  -webkit-appearance: none
+
+// stylelint-enable