Maintain dimensions when diff frame is loading

Instead of collapsing the diff while loading the next one and
expanding it again afterwards, just dim it (lower opacity and
disable pointer events), and when ready either close it or undim
it.

This should reduce flickering, especially during MassPatrol.

Also changed closing/opening animation to use CSS3 transitions
with Y-scale and max-height (instead of jQuery fadeIn/fadeOut
with opacity and display none/block).

Fixes #25