[IMP] website: improve install theme layout

addons/website/__manifest__.py

addons/website/models/res_config_settings.py

addons/website/static/src/img/phone.png

addons/website/static/src/js/theme_preview_form.js

addons/website/static/src/js/theme_preview_kanban.js

addons/website/static/src/js/widget_iframe.js

addons/website/static/src/scss/website.backend.scss

addons/website/static/src/xml/theme_preview.xml

addons/website/static/src/xml/website_widget.xml

addons/website/views/website_templates.xml

addons/website/views/website_views.xml