fix #480: scrollToBottomAnimated: fix long message handling

[toblerpwn]

fixes #480 - tested in a few other scenarios, and behavior was as expected. should probably be tested further against common layout-style breaking points, however.

[toblerpwn]

Correction: in some cases, Top alignment is best. 6a04d75 handles both cases.

[toblerpwn]

As was pointed out by @galambalazs in #480 , this PR improves the handling of large cells, but it is still imperfect; although huge cells are shown and scrolled properly to the their bottom, there remains a non-beautiful margin between the bottom (huge) cell and the input toolbar. Ideally we would have a perfect margin.

I would still suggest merging this PR - since it improves on the current state of the project - until a more robust fix can be implemented.

[jessesquires]

thanks @toblerpwn ! i'll look at this soon

[toblerpwn]

Cool @jessesquires - no rush per se, I'm just running a custom branch 'til these fixes sort themselves out in the main pod one way or another. :)

I've got a couple more things coming your way, too! Boo yah!