Add ability fold close/open (minimize) files on pull request views. (BB-12809)

When you are viewing a pull request, the page can get very long. Often times, there are also ling individual changes that could be folded up to save space and time. For example, someone could delete some large vendor file tha tis thousands of lines long. I know this change is ok but yet I need to scroll many times to get past it. It would be much easier if I could just fold that file up to get it out of the way..