prettier: Add to CI to ensure we stay clean.
prettier: Add to CI to ensure we stay clean.

Now a change which breaks our formatting will fail in CI, so we can
fix it before the PR is merged rather than have to periodically clean
things up in retrospect.

While we're here, simplify our `prettier` command line a bit by using
the built-in glob feature.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile