Add xstatic and xstatic-jquery for Horizon

We are unbundling the static JS files from Horizon, moving them
to separate packages where possible. That means, that we have to
depend on those packages in requirements now.

This is for the patch https://review.openstack.org/#/c/82516/

Both xstatic and xstatic-jquery are not yet packaged in Ubuntu or
Fedora, but are used by at least one other large project, MoinMoin 2.

Change-Id: Iac5b5baf9d46c8deffd51d8b33345f7e21a50126
Partial-implements: blueprint https://blueprints.launchpad.net/horizon/+spec/remove-javascript-bundling