Wraps long lines in bulk email messages.

RFC 2822 states that line lengths in emails must be less than 998.
Some MTA's add newlines to messages if any line exceeds a certain
limit (the exact limit varies). Sendmail goes so far as to add
'!\n' after the 990th character in a line. To ensure that bulk
mail messages look consistent long lines are wrapped to a
conservative length.

LMS-1466
Merge pull request #3455 from edx/usman/lms1466-email-formatting

Wraps long lines in bulk email messages.