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.
I have to admit, I'm not the best at URL-/entity-encoding either, but they are definitely not the same.
HTML entities look like `&amp;` (`&#38;`) or `&quot;` (`&#34`), while special chars in URLs look like `%25` or `%22`.

But as I just looked through regisitry.json, there's not a single case where URL params or similar are used, so well, I'm fine with the current solution even while that probably won't work for edge cases (btw, it's the same we do inside Brackets). We just need to make sure no quotation marks, neither `'` nor `"`, can "break out" and cause XSS.