[IMP] all: new design to match the one on odoo.com

* upgrade to bootstrap 3.3.6

* new Header, port website LESS
  - Add the new header
  - Modify sub-header in order to handle “back” button and version
    switcher
  - Port website’s Less:
    * Use same variables’ names
    * Review fonts and colours in order to fit the new style

* add new footer

* Review and optimise JS code in order properly handle the new header,
  footer and card.top animations

* replace  banner images

_extensions/odoo/static/bootstrap-3.3.6/less/mixins/responsive-visibility.less

@@ -0,0 +1,15 @@
+// Responsive utilities
+
+//
+// More easily include all the states for responsive-utilities.less.
+.responsive-visibility() {
+  display: block !important;
+  table&  { display: table !important; }
+  tr&     { display: table-row !important; }
+  th&,
+  td&     { display: table-cell !important; }
+}
+
+.responsive-invisibility() {
+  display: none !important;
+}