Instructor dashboard no pending task message

This commit adds a message to the instructor dashboard that is displayed when
no instructor tasks are currently running. This message is displayed where the
instructor pending tasks table would normally be placed, and is replaced
with the table when there are running tasks.
Actually - instead of putting the same text in 5 templates, I think it would be better to do it in `util.coffee`. I believe you can do something like
```coffee
@$running_tasks_section.hide()
@$no_tasks_message.text gettext('No tasks currently running.')
```