[FIX] website: use absolute href location after page url change

Before this commit:
When editing a page URL through page properties it would redirect to that new
url afterward. If the url was not starting with '/' and had multiple level
before and after edition (eg: page/action to page/realaction) it would then
redirect to a wrong relative URL.

Now, we ensure that we redirect to the correct URL: the one slugified and
cleaned in the backend.

Step to reproduce:
  - Create a page '/page/action'
  - Go to that page and open Page Properties
  - Edit the URL to 'page/someaction'
  - You will be redirected to the incorrect relative URL /page/page/someaction

addons/website/models/website.py

@@ -715,7 +715,7 @@ class Page(models.Model):
                 'website_id': website.id,
             })
 
-        return True
+        return url
 
     @api.multi
     def copy(self, default=None):

addons/website/static/src/js/menu/content.js

@@ -198,7 +198,7 @@ var PagePropertiesDialog = widget.Dialog.extend({
             kwargs: {
                 context: context
             },
-        }).then(function () {
+        }).then(function (url) {
             // If from page manager: reload url, if from page itself: go to (possibly) new url
             if (self._getMainObject().model == 'website.page')
                 window.location.href = url.toLowerCase();