Convert relative URLs to absolute URL.

[na9da]

Now that new routing changes have landed, relative URLs in catalog definitions will break.

• We need to auto-convert relative URLs to absolute ones
• Maybe also accept, --base-url param to set base URL to something other / (eg: https://nationalmap.org.au/renewables)

[soyarsauce]

This is easily resolved, relative URLs will continue to work if terriamap/terriajs is configured correctly with the base href 🙂