Allow admins past license expiration screen in order to easily update key

Created by: slimsag

Prior to this change, if your trial license expired then all users including the site admin would be locked out of Sourcegraph, making it impossible for many site admins to update their license key without resetting their management console password first (see #5559 (closed)):

After this change, site admins bypass this hard-lockout (plain users are still hard-locked) and instead are shown a warning alert informing them all users have been locked out and they need to update their license. This allows them to retrieve their management console password from the site admin area easily:

Accepted risk with this approach: A site admin could theoretically promote all users to site admins and use JS to hide the alert, thus bypassing our license lockout mechanism. This seems low-risk and unlikely in practice. If this becomes problematic, we could enforce e.g. only the first 5 admins can bypass into the app.

Fixes #5559 (closed)