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

added the execution time activity_listing.html
@bobsilverberg .ready for review
The indentation is still wrong for this line. Take a look at this diff in Github and you'll see. I think this issue is that there are some tabs in the line, not just spaces. You should always use spaces for indenting, never actual tabs. You should be able to configure your editor to always use spaces.

Also, your use of the `|lower` filter is causing the label to be lowercased too, so it is appearing as `date completed` instead of `Date completed`.

Also, this is showing up for all attempts, regardless of status, which is ok, but it means the label is wrong when the status is not `Finished`. For example, the label still shows `date completed` even if the attempt was Abandoned or only just Started. I think it would be better to create the label from the current status of the attempt, which would ensure that it is always correct.