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

static/assets/bootstrap/sass/mixins/_text-hide.sass

@@ -0,0 +1,11 @@
+// CSS image replacement
+=text-hide($ignore-warning: false)
+  // stylelint-disable-next-line font-family-no-missing-generic-family-keyword
+  font: 0/0 a
+  color: transparent
+  text-shadow: none
+  background-color: transparent
+  border: 0
+
+  @if $ignore-warning != true
+    @warn "The `text-hide()` mixin has been deprecated as of v4.1.0. It will be removed entirely in v5."