docs/website: consolidate CDN's and enable SRI

Some of our cdn's are going discontinued (rawgit) and some others are
not recommended anymore, thus we update to the recommended cdnjs.
This patch enables also SRI protection on js to be sure the modules we
download are not manipulated in any way.

About SRI:
https://developer.mozilla.org/en-US/docs/Web/Security/Subresource_Integrity



Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Angelo Compagnucci <angelo@amarulasolutions.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>