add 'more info' to extension listings (both html and css)

Let me know if the format/conditionals are okay, or if you want to check that a homepage exists in the .json files through the .js files.
You should probably [`htmlEscape`](https://github.com/adobe/brackets-registry/blob/master/lib/registry_utils.js#L38) the link and then use 3 curly brackets (`{{{metadata.homepage}}}`)