[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/opacity.less

@@ -0,0 +1,8 @@
+// Opacity
+
+.opacity(@opacity) {
+  opacity: @opacity;
+  // IE8 filter
+  @opacity-ie: (@opacity * 100);
+  filter: ~"alpha(opacity=@{opacity-ie})";
+}