Bug 1032371 - Display users who have completed task on task detail page

added the execution time activity_listing.html
@bobsilverberg .ready for review
This `<div>` isn't properly indented. You'll notice that it isn't lined up with the other `<div>`s on the page. 
Also, there should be a space between the double curly braces and what is inside them (again, see the other code on the page), and there should be a space between `Date completed:` and `{modified}`.