Hide completed tasks after 5 seconds

# What's up

After completing a task, the task was left in the page. This PR hides the task after 5 seconds, using `Ember.run.later` instead of `setTimeout` because it is more efficient. Doc says:

<blockquote>
Invokes the passed target/method and optional arguments after a specified period if time. The last parameter of this method must always be a number of milliseconds.

You should use this method whenever you need to run some action after a period of time instead of using setTimeout(). This method will ensure that items that expire during the same script execution cycle all execute together, which is often more efficient than using a real setTimeout.
</blockquote>