Message.add_props: Clean up variables.
Message: stop setting message_style attribute.
Message.add_props: Don't use c.user.

Use `user` which is passed into the function.
Message.add_props: Clean up batch lookups.
Message.add_props: Batch lookups of override to and author.
Message.add_props: Run set_unread in batch.
Message.add_props: Use mods_by_srid to find user moderated subreddits.
Message.add_props: detangle logic.
Make messages from user as subreddit more visually distinct.

Previously they looked like a regular message sent from the current
user. Now they indicate they are sent via a subreddit.
Messages: always show "from" first.
Message: don't distinguish author of parent comment.
Update thing.recipient to thing.user_is_recipient in message.compact.