Improve certficate warning during first setup

Currently, users see an Invalid Certificate when they connect to FreedomBox for the first time. What we can do is have redirection disabled by default and then enabling after setup process is completed. This can happen at two steps:

• We can enable HTTPS redirection after Let's Encrypt is successfully is setup. However, this has the problem that users who don't obtain the certificates due to whatever reason (such as when using on Tor based services) will be at risk. Also admin account creation over plain HTTP is risky. So, we should not likely do this.
• We can enable HTTPS after showing an extra first boot step that says that they will now be turning on HTTPS and show an animated page that shows on how to deal with the certificate warning.

Related issue: #1586.