Before this patch, when a user dragged a fixed snippet on the email body, the result had some attributes that are internally used in Odoo to know if you are replacing a part of a view, and which.
This made that editing the placeholders changed directly the snippet template itself.
On other cases, when this addon was removed and then reinstalled, it made that previous templates raise a MissingError when saving any changes on them.
Chances are that if you have (maybe unnoticedly) hit this bug, you have some snippet XMLID from this addon marked as `noupdate=1`, so the recommended upgrade path is to uninstall this addon and reinstall it.
This commit extract the vertical resizing plugin part from the
`website_mail_snippet_responsive` module into its own module, to allow
reusing it easily.
Responsive is a hard thing in mails, because mail clients usually render very badly everything.
This new module adds a good collection of snippets in a pseudoresponsive manner, that will seem responsive in most cases, although they are not, technically.
Inspired by https://css-tricks.com/ideas-behind-responsive-emails/.
Jairo Llopis
Daniel Gómez-Zurita
Sergio Incaser