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.
i18n (sorry just noticed!). Here you must used ngettext.
```python
from django.utils.translation import ungettext
num_emails = task_output['succeeded']
number_sent = ngettext(
    "{num_emails} sent",
    "{num_emails} sent",
    num_emails
).format(num_emails=num_emails)
```

I know it doesn't make a ton of sense to use ngettext when the strings are identical, but in other languages, the strings may not be identical.