Add the ability to Share pull requests

Need an intuitive way of getting a link to the section including a diff and comments in the pull request screen. When you have a large pull request containing hundreds of diffs and want to IM about a specific diff, have no way of linking to target of specific diff. Can't just use the built-in commenting functionality because notifications are unfriendly to non-threaded-email notifications in a situation with lots of files in a pull request and lots of comments (STASH-2839). Please add a share link that maybe unhides a div containing a small area containing the shortened URL (and perhaps provide Jabber, etc. support to share) or just a link to target that part of the page. Thanks.