Bug 1091441 - [Messages] the thread view is flashing while loading if there are MMS

- Use promise.all to scroll after all the mms contents parsed and attached
- Fix the chunk bug
Hah, we didn't respect ```number``` parameter before, good catch! 

Here ```i``` can be less than zero (eg. in case we want to show new chunk of 10, but we have only 8 hidden items), we have to handle this case too. Maybe ```Array.slice(ThreadUI.container.getElementsByClassName('hidden'), -number)``` or even ```Array.slice(ThreadUI.container.getElementsByClassName('hidden'), -number).reverse().forEach()```. The latter is not really efficient though, but maybe it's not that critical.