The canary favicon and "Next" badge can be inconsistent sometimes

The favicon is rendered based on the server-side ENV['CANARY'] while the "Next" badge is rendered based on the cookie.

There are cases where these can have different values:

1. Cookie is set but canary is disabled / in maintenance mode so non-canary server handles the request
2. Cookie is not set but browsing the gitlab-org group where we force canary

I think both of these should be based on the same thing and I think server side flag is more accurate.

Using the server-side flag and adding a conditional on the HAML would also fix #28801 (closed) 😄