Message.add_props: _force_unicode body when adding warning.

This was causing UnicodeDecodeErrors.
Stop converting multis with friends or all to special subreddits.
Filter out bad Subreddit names from MultiReddit.

Previous method was letting non-ascii characters through in the
MultiReddit.path which caused issues in other areas.
Traffic processing: Switch from m1.xlarge to m3.xlarge.

The new instance type is faster and cheaper.