Added Functionality which helps to sort PR's by recent activity.
    
    Changed /lib/__init.py__ to accept order in which the PR's are to
    be sorted from requests.html, so that if either Opened/Modified
    is clicked, the PR's are sorted accordingly with an arrow symbol
    beside them describing the way they were sorted.
    
    Fixes https://pagure.io/pagure/issue/1404
    Merges https://pagure.io/pagure/pull-request/3109