Draft: Add a basic disclaimer about review sites

This adds a basic (unstyled) warning above the navbar when LEKTOR_IS_REVIEW_APP=1, to let users know if they're browsing a review site.

The warning currently looks like this:

@emmapeel How does this look to you? It has the link and it's wrapped in a translation call. I checked, and this.url_path should properly link to the correct page if the user is browsing in a language other than english. My one concern is that it might not be eye-catching enough.