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

static/assets/bootstrap/sass/mixins/_caret.sass

@@ -0,0 +1,56 @@
+=caret-down
+  border-top: $caret-width solid
+  border-right: $caret-width solid transparent
+  border-bottom: 0
+  border-left: $caret-width solid transparent
+
+=caret-up
+  border-top: 0
+  border-right: $caret-width solid transparent
+  border-bottom: $caret-width solid
+  border-left: $caret-width solid transparent
+
+=caret-right
+  border-top: $caret-width solid transparent
+  border-right: 0
+  border-bottom: $caret-width solid transparent
+  border-left: $caret-width solid
+
+=caret-left
+  border-top: $caret-width solid transparent
+  border-right: $caret-width solid
+  border-bottom: $caret-width solid transparent
+
+=caret($direction: down)
+  @if $enable-caret
+    &::after
+      display: inline-block
+      width: 0
+      height: 0
+      margin-left: $caret-width * 0.85
+      vertical-align: $caret-width * 0.85
+      content: ""
+
+      @if $direction == down
+        +caret-down
+      @else if $direction == up
+        +caret-up
+      @else if $direction == right
+        +caret-right
+
+    @if $direction == left
+      &::after
+        display: none
+
+      &::before
+        display: inline-block
+        width: 0
+        height: 0
+        margin-right: $caret-width * 0.85
+        vertical-align: $caret-width * 0.85
+        content: ""
+
+        +caret-left
+
+    &:empty::after
+      margin-left: 0