Thread message feed staying focused at the end of conversation

Currently, when new messages come through, the scroll of TMF doesn't change (e.g. when the TMF is scrolled down to the last message, new messages in the thread load hidden below the bottom edge of TMF).

The behaviour is implemented for MMF in rollers.js.

• when TMF is scrolled all the way to the bottom, scroll with the new messages (keep being scrolled to the bottom)
• when TMF not scrolled to the bottom, keep TMF as is and just put the new message at the end of the TMF without any scrolling