Email content history fix and additions

This addresses a bug in the email content history table where
"Unknown" was displayed in the number of emails sent column if any sort of
failure occurred during email sending. This behavior has been edited so now
the number of emails that failed to send is displayed, along with the number
of emails that were successfully sent. If the email task is still pending,
"0 sent" is displayed.

As a small addition, the table now also includes the authors of previously
sent emails, and the modal window for an email also displays its author.
@stvstnfrd well, I would disagree with this.

The username in this column represents merely the person who was logged in when the email was sent. Thus I object to the term "Author" as they may have not really been the "author" of the email - maybe it was collaboratively done and just this person just was logged in.

I have some reservations about the term "From" - I see where you're coming from, but I do not believe the majority non-technical course staff users (at least on edx.org) will understand the distinction between "From" and "Sender". In fact until I read this I didn't either. I assumed that what we did in https://github.com/edx/edx-platform/blob/master/lms/djangoapps/bulk_email/tasks.py#L370 was constructing the "From" field.

I would like to defer to our documentation and possibly UX team in this situation. I want to make this page easy to use and understand to a non-technical audience, even at the risk of being semantically incorrect.