[IMP] website: stay in backend on F5 or CTRL-R while in website preview

Since [1], it was no longer possible to refresh the page and stay in
the backend at the same time. This was known at the time and was an
accepted downside.

However, with this commit, a compromise has been found, if the keyboard
shortcut for a refresh (CTRL-R or F5) is used while the Website Preview
is shown (mounted), then the website preview will do its best to keep
the user inside the backend.

This will make developing features for the website modules easier, as
before, every refresh would require an extra click to return to the
backend.

To note: the debug=[mode] param would also be lost
on refreshing a website page. Starting 16.4, since [2], this would make
tracebacks harder to read as for every refresh, they would no longer
point to the source, rather just the minified file. Therefore, with
this commit, we also keep the debug mode in the URL when doing a
keyboard refresh.

[1]: https://github.com/odoo/odoo/commit/1c18b79972c3b0a97197b98390e0ba9fda703585
[2]: https://github.com/odoo/odoo/commit/59f49d6a8c856a5d8ca28dcfd7840baaa1c7672f



task-3458691

closes odoo/odoo#131236

Signed-off-by: Romain Derie (rde) <rde@odoo.com>