Use the MessengerBot's object rather than the current user's when generating preview HTML

The easiest way to test this is previewing "~~~~". Previously this would have
shown the current user's signature, now it will show the bot's signature.

Change-Id: I744042d551b253b56dcc9f23a93a7632e4830deb
Merge "Use the MessengerBot's object rather than the current user's when generating preview HTML"