Add a warning alert when usage exceeds the limit of free user accounts

Created by: dadlerj

A new non-dismissable alert is now shown to all users on an instance when there's no valid license in use, and the count of users exceeds the free user limit (20, see https://github.com/sourcegraph/about/pull/104, as well as https://docs.google.com/document/d/1L72XF8EY5i0QL8b338j0g7ERrgNruXDvvhZjadcOFZc/edit#, internally).

The hard pay wall now appears at 50 users, rather than 100 (this may be a pain for some users — we should update the paywall page to make it more obvious how to mitigate the issue (e.g. a link to the management console docs if there isn't one, a link to get a free trial, etc.).