Task Completion

A short gif showing task completion and un-completion (?). When a task is marked as completed, it shifts itself further into the background, and out of the way, while still remaining accessible.

I found that when I was building this feature, I'd sometimes mis-click a task, and it would immediately go away, which meant I had to open the Todoist app and mark it back to incomplete. In this rev, the task still goes away, but requires a refresh to be removed from the list.