Improve automatic external_url pre-populating

Depending on the server setup, hostname can be without http(s) which again creates an issue during installation.

After talking with @sytses we should fix this rather than removing it. This is because this feature considerably eases initial installation.

@jacobvosmaer I will create a function that will do a regexp on an external_url and add http to it if it doesn't exist. I wanted to do a curl but then I realised that external_url is not reachable at that point still.

Let me know if you have a better idea.