Don't show CTAs on demo instance

Created by: muratsu

When we visit demo.sourcegraph.com today, we see all the CTAs that a new user sees. These CTAs, while beneficial for new users, don't look great on demos/videos. If we can have a feature flag for the demo website and hide all the CTAs that would help the partner teams greatly.

More context: https://sourcegraph.slack.com/archives/C03CSAER9LK/p1651070583780779