[FIX] website_theme_install, *: multi-website customization dialog

* theme_bootswatch, theme_default

Before this commit, the customization dialog for themes was not
multi-website. That means if a theme was installed on a website, the
customization dialog was enabled for all the websites.

Note: this commit uses the '_post_copy' system of theme installations
for the first time and it had to be adapted a little bit.

addons/theme_default/data/theme_default_data.xml

@@ -9,8 +9,4 @@
         <field name="action_id" ref="action_website"/>
         <field name="state">open</field>
     </record>
-
-    <record id="website_theme_install.customize_modal" model="ir.ui.view">
-        <field name="active" eval="False"/>
-    </record>
 </odoo>