Hide sandbox version of website

[gusthoff]

As reported in #1201, the sandbox version of the website shows up when searching for the learn website. We can attempt to avoid this by introducing a robots.txt file, which asks the search engine to not index the website.

[gusthoff]

Unfortunately, there isn't any easy solution to prevent the website from being indexed. However, a simple authentication system was introduced, which redirects to the main website if the visitor doesn't provide the correct password.