Bump jquery to 1.12.4 (#236)

This will allow us to migrate to the latest version of jquery 1.x series.

After we're on 1.9.x we can upgrade to the 3.x series.

I bumped this, and then checked the developer console while clicking
through the various panels offered in FDBT... Everything worked as
expected and I got zero warnings from `jquery-migrate`. So I think it's
safe to pull this in.

This is a pre-requesite to upgrading to the latest version of `jquery`,
as per their docs we have to first bump to `1.9+`, then bump the version
of `jquery-migrate`, then can bump to latest `jquery` version.

While I could do this in a single PR, I thought simpler to do it as a
series of PRs for easier reversion if we later run into problems.

Related:
* #166