Change default for jQuery CDN to false

This changes the default setting for using jQuery.com's CDN from true to false. This is in response to the vote in [this Loomio proposal](https://www.loomio.org/d/zogZmP8d/disable-jquery-cdn-by-default?proposal=gdzIi0Ye).

Note: I think this shouldn't be merged until the next major release is being prepared, so that podmins are more alert to the possibility of defaults being changed. So please set a flag for next-major, and I'll have to rebase when the time comes.

Hope that's OK.