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