Fix authz license check

Created by: beyang

This addresses https://github.com/sourcegraph/sourcegraph/issues/1284 and reverts https://github.com/sourcegraph/sourcegraph/pull/1236.

Note, there is a behavior change: previously, the license requirement was enforced a user attempted to sign in via GitHub or GitLab. That didn't really make sense, because what we really want to enforce is the use of GitHub/GitLab repo permissions. Now there is not really an obvious place to hard-enforce that requirement. What it currently does is show a warning message to the admin user if they are using the feature without a license, but it does not actually disable authz checks (it seems bad to remove authz checks or render the instance unusable if the license is removed or inadvertently changed). If that sounds good, this should be mergeable as-is. Otherwise, what is the desired behavior?