Display toolbar based on the presence of a cookie.

This small change allows developers to only see the toolbar when they have a specific cookie set.

I have an app in a preview stage where there are some "regular" users that should not see the toolbar, but developers should still be able to. I didn't want separate deployments so I coded this in.

Works like so:
1. Set 'DEBUG_TB_COOKIE' to the name of the cookie you want to check for (e.g. `let_me_debug`).
2. Set a cookie for your site in your browser, e.g. `document.cookie="let_me_debug=1"`