[IMP] website: button to easily back to page manager from form view

Before this commit:
When you were on the page manager and you were editing a website.page's
content, it would redirect you to the website.page's ir.ui.view's form view in
the backend.
From there, you would'nt have an easy way to get back to the page manager:
your only option was to go to the website module and click on 'go to website'
and then click on menu/manage pages.
It was not convenient when you were editing multiple page quickly or just
taking a look into some pages.

Now, you have a button on the ir.ui.view's form view to get directly on the page
manager.

Note: this back button is obviously invisible for ir.ui.view not related to a
website.page

addons/website/models/ir_ui_view.py

@@ -20,6 +20,13 @@ class View(models.Model):
 
     customize_show = fields.Boolean("Show As Optional Inherit", default=False)
     website_id = fields.Many2one('website', ondelete='cascade', string="Website")
+    page_ids = fields.One2many('website.page', compute='_compute_page_ids', store=False)
+
+    @api.one
+    def _compute_page_ids(self):
+        self.page_ids = self.env['website.page'].search(
+            [('view_id', '=', self.id)]
+        )
 
     @api.multi
     def unlink(self):
@@ -142,3 +149,11 @@ class View(models.Model):
             return lang_code
         else:
             return super(View, self).get_default_lang_code()
+
+    @api.multi
+    def redirect_to_page_manager(self):
+        return {
+            'type': 'ir.actions.act_url',
+            'url': '/website/pages',
+            'target': 'self',
+        }

addons/website/views/website_views.xml

@@ -209,12 +209,17 @@
 
             <field name="inherit_id" ref="base.view_view_form"/>
             <field name="arch" type="xml">
-
                 <field name="name" position="after">
                     <field name="website_id" readonly="1"/>
                     <field name="key" readonly="1"/>
+                    <field name="page_ids" invisible="1" />
                 </field>
-
+                <sheet position="before">
+                    <header>
+                        <button name="redirect_to_page_manager" string="Go to Page Manager"
+                            type="object" attrs="{'invisible': [('page_ids', '=', [])]}"/>
+                    </header>
+                </sheet>
             </field>
         </record>