Bug 1087266 - Add datetime for task execution in Task Activity view

added the execution time activity_listing.html
@bobsilverberg .ready for review
@tessie The `<div>` is still not lined up with the `<div>` above it. It would be very helpful if you would check for these indentation issues before submitting the pull request so I don't have to keep pointing them out to you every time.

Also, you are missing the closing `</div>` tag.