Easyfix #3363: displaying hashtag anchor in url when file link clicked, and scroll with appropriate spacing
Changes

- Adjusted "DIFF_SCROLLDOWN_AMOUNT" value to 15 from original 100, which reduces the spaces between top of the element to the browser boundary after jumping to the anchor, avoid leaving a large gap between

- setting the location.hash with the anchor value at the time of scrolling, to replace the part being stripped out by backbone

manage.py test passed
created open reviews with multiple changed files and tested clicking on file links to confirm the url is displaying the correct hashtag and having no extra spaces between the tagged element and top of the browser
opening new page with link including the hashtag to confirm the hashtag remains in place, and the page is scrolled to the correct tagged element location