Before this commit, some snippets did not have any or complete preview
when dragging them over a dropzone.
This commit covers `s_website_form`, `s_countdown`, `s_map` and
`o_facebook_page`.
Some elements with class `s_preview` are added to the template. These
elements are removed when the snippet is dropped.
Also, the class o_colored_level was added to snippets so that the
previews matches the snippet dropped.
For "New page templates", an `s_keep_preview` class can be added on a
parent element of an `s_preview` to prevent its removal when creating
the page - thereby making the snippet ready as if is had been dropped
into the page.
Note: `s_chart` and the "dynamic snippet" and its variants are not
covered in this commit.
task-3555325 (was task-2431469)
Part-of: odoo/odoo#138748
Co-authored-by: Benoit Socias <bso@odoo.com>
