[FIX] website: truncate description at 160

Search engines (or one at least) truncates the description at 160. The advised
size is between 150 and 160 so no need to force a smaller size. It's a good
idea to keep it at 155 but larger sizes should be allowed.
cf: https://moz.com/learn/seo/meta-description

We could fully remove the limit as there is corner cases where a longer size is
used (https://moz.com/blog/i-cant-drive-155-meta-descriptions-in-2015) but this
should be in addition with an indication for the user of the current size and
the advised size.

addons/website/static/src/js/website.seo.js

@@ -354,7 +354,7 @@ var Configurator = Widget.extend({
     canEditKeywords: false,
     canEditLanguage: false,
     maxTitleSize: 65,
-    maxDescriptionSize: 150,
+    maxDescriptionSize: 160,  // TODO master: remove me and add warning
     start: function () {
         var self = this;
         var $modal = self.$el;