Remove jquery, use fetch for AJAX

[machawk1]

Various DOM selectors $() are also much better supported in browsers nowadays, too. The more native JS we can use, the less outdated the dependencies contained.

[machawk1]

There is a caveat in that the Miller columns library we use is jQuery-based.

[machawk1]

Revisiting this after some exploration, there still does not seem to be a decent Miller columns JS library not reliant on jQuery.

[machawk1]

https://github.com/alphagov/miller-columns-element is an implementation of Miller columns using Custom Elements w/o the reliance on jQuery.

[machawk1]

https://github.com/sachinchoolur/replace-jquery