Added a filter on the users page for viewing inactive/removed users

Review Request #358 — Created Jan. 25, 2017 and updated — Latest diff uploaded

Review Board
There's been a request to add a filter to the Users page that allows for showing those who are no longer part of the team. This was done by updating class 'UsersDataGrid' to introduce show/hide inactive users functionality. I also created templates/datagrids/user_listview.html to display the show/hide inactive toggle.

Testing Done:
This was tested by changing a user from active to inactive via admin page. Then, I checked on the users page if an inactive user disappears after clicking 'Hide inactive', and whether an inactive user also appears after clicking 'Show inactive'.

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