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

added the execution time activity_listing.html
@bobsilverberg .ready for review
This should not be two calls to the `_()` method, it should just be one. Something like this:

```python
{{ _("Date {state}: {modified}")|fe(state=attempt.get_state_display(), modified=attempt.modified.strftime('%Y-%m-%d: %I:%M:%S %p').lower()) }}
```

Also, I know this is code that was there before you made this change, but we should be using single quotes `'`, not double quotes `"` everywhere in Python code except where double quotes are needed.
Also, I hate to say it but the indenting in this file is not correct.