docs/website: fixing js for the new feed source

Feed source is moved to a local atom file available on the web server to
remove further problems with unreliable feed sources.
Moving the feed required a bit of refactor of the load_activity function
so it won't download two times te same url.

This change requires a cron job like this enabled on the website:
* * * * * wget -O new.atom https://lore.kernel.org/buildroot/new.atom



Signed-off-by: Angelo Compagnucci <angelo@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>