Add hashes to JS and CSS files for cache invalidation

Only do it for JS on production as requirejs makes this hard on
development and it shouldn't really be needed there either.

When I say it makes it hard I mean that because it's requesting lots of
files and then using the path of those files as the module name putting
in place the code to resolve this is both painfully manual and hence
error prone and also problematic because of the way require handles
fetching. Given cache busting is only really an issue with production
deployment it's not worth the pain.

For #554
remove entity-save-changes class from person submit button

the enter-to-save augmenter triggers a click on anything with a class of
entity-save-changes when you hit enter in an input box. If the submit
button has this class then it means there are two items with that class
on the page which means that sometimes the form is submitted twice. This
can cause multiple memberships to be added.

Fixes #599