closes https://github.com/TryGhost/Team/issues/2206
- removes `www.` from the url shown on links table in post analytics
- we had previously removed http(s) protocol from it as well, and they are only shown while editing the url
no issue
- pattern of downloading a file by creating an iframe and setting the `src` attribute was repeated throughout the codebase and was using a mix of native and jQuery patterns
- added a `utils` service for housing one-off utility methods like this to avoid repetition and mixed patterns becoming more widespread (we want to get rid of jQuery usage)
