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.
Strictly speaking, it's correct to entity-encode & in URLs inside an href attribute - that's just normal HTML entity encoding, and the browser should automatically decode it when parsing the HTML. However, that's what `{{}}` should already be doing, so it isn't necessary to call `htmlEscape()`.

If you're thinking that it should be URL-encoded, I think you can just use the normal JavaScript `encodeURI()` function for that. I think it's ok for it to be both URL- and entity-encoded - entity-encoding is decoded by the HTML parser first, and then URL decoding happens when the user clicks on the link and the link is interpreted by the browser during a navigation.

However, I think it's probably better to just assume that URLs in the package.json are already properly URL-encoded - I suspect that's generally how things are assumed to work, and we don't want to double-encode something that's already URL-encoded.