[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/nav-divider.less

@@ -0,0 +1,10 @@
+// Horizontal dividers
+//
+// Dividers (basically an hr) within dropdowns and nav lists
+
+.nav-divider(@color: #e5e5e5) {
+  height: 1px;
+  margin: ((@line-height-computed / 2) - 1) 0;
+  overflow: hidden;
+  background-color: @color;
+}