Revert about.sourcegraph.com redirect changes from PR #23983
Created by: bretthayes
This reverts changes made in #23983 and re-implements the redirect to about.sourcegraph.com if users are logged in.
This closes sourcegraph/about#5284.
Test plan
- The client redirects non-logged-in users to
about.sourcegraph.com
- The client redirects logged-in users to
/search
- The server redirects logged-in users to
about.sourcegraph.com
, otherwise do nothing - The Go test passes