Add ability for authorized users to enter track URL from disk review page if it is empty

[eric-gilbertson]

Add ability to enter track URL from disk review page if it is empty. Currently users must click through several pages to enter a URL. Allowing entry directly from the review page would greatly simplify this process. Changes include the following

• show edit pencil icon in place of play icon if track has no URL and user is logged in
• display URL entry modal when edit icon is clicked
• upon save update the URL for the track