Shift Complete Item Functionality to Task Dropdown Menu

[tjfwalker]

Current complete item behavior —as implemented in #9— is tied to the wrong UI component as a stop-gap. This task is to move that behavior to the correct place —the item dropdown menu— as made possible by #129.

[tjfwalker]

@jrob8577 please add this to the 'Current Week' project board and assign @shakalee14 and I (@tjfwalker).