Fixes tabs appearing in the subject line of emails.

Python 2.7 has a problem that causes tabs to be used to break up long headers
instead of spaces. When Review Board sends an email with a Subject longer than
62 characters, it inserts a tab to seperate the text into mutiple lines.

Now instead of the Subject header being seperatated by tabs it is seperated by
spaces.

Testing Done:
Ran unit tests.

Bugs closed: 3613

Reviewed at https://reviews.reviewboard.org/r/6647/