cockpit: Add information on how to access the app

When Cockpit is accessed using an IP address, it does not work and shows a blank page after the login screen. To prevent users from accessing Cockpit using an IP address add more information to the Cockpit application description.

• Add a third paragraph to the Cockpit application description with the following content: "Cockpit requires that you access it through a domain name. It will not work when accessed using an IP address as part of the URL."
• Add a new section with title "Access" that contains the following information. A paragraph that says "Cockpit will only work when accessed using the following URLs." This should be followed by a list (<ol>) of URLs. These URLs are constructed using the template <origin>/_cockpit. List of origins should be retrieved from Cockpit's configuration file /etc/cockpit/cockpit.conf by using the already existing utility get_origin_domains() and filtering out localhost URLs.

Original description

Jesse Smith observed the following in his review of FreedomBox:

"The Cockpit module installed, and when I launched the application I was taken to Cockpit's login screen. After entering my username and password, I was shown a blank page. I think Cockpit was the only application which did not work properly during my trial. I tried using Cockpit in a few different browsers and each produced the same blank page after I had successfully signed into the module."

Source: https://distrowatch.com/weekly.php?issue=20190930#freedombox

This problem has been reported by others recently, but I was surprised to see that it didn't yet have an issue on gitlab.