[REF] website: install theme_default on first website by default

The original goal was to not have any pre-installed theme by default
and to force the user to consider installing one through the customize
dialog.

Now that we have a system to allow switching theme, it is better to have
one installed by default and allow the user to switch theme from the
customize dialog (note that last part was made in the parent commit with
the merge of website_theme_install into website).

Note: theme_default is purely empty, it is only installed by default for
clarity in the theme selection screen, for the default website.

addons/theme_default/__manifest__.py

@@ -8,11 +8,13 @@
     'sequence': 1000,
     'version': '1.0',
     'depends': ['website'],
-    'data': [
+    'demo': [
+        'data/theme_default_data.xml',
     ],
     'images': [
         'static/description/cover.png',
         'static/description/theme_default_screenshot.jpg',
     ],
     'application': False,
+    'auto_install': True,
 }

addons/theme_default/data/theme_default_data.xml

+<?xml version="1.0" encoding="utf-8"?>
+<odoo>
+
+<record id="website.default_website" model="website">
+    <field name="theme_id" ref="base.module_theme_default"/>
+</record>
+
+</odoo>