Add a difficulty column to filter+table for Task Overview

I'm not sure that, for filtering, the display of "1.5) medium" looks so good compared to the others; maybe ```source.priority * 2 - 1``` would look better at L1237? This might need to be updated when https://trello.com/c/4C8w1z5h/17-task-difficulty-settings-v2-priority-multiplier changes.

Also, please ignore the 1750/1776 change; must've been an errant keystroke while looking.