Add alerts to site-admins when license keys are about to expire

Created by: dadlerj

Addresses https://github.com/sourcegraph/sourcegraph/issues/3016 Addresses first item from https://github.com/sourcegraph/sourcegraph/pull/2894

Instead of making it a non-dismissable alert, I made it re-appear every day from 7 days out.

While this example shows "7 months", it really will only appear when there are 7 or fewer days left.