This patch improves the Post and Shrine transitions by making the AppBar
into a Hero and changing the default MaterialPageTransition. Now the
AppBar transitions smoothly between screens and the
MaterialPageTransition doesn't involve a fade effect.

Also, rejigger the bounds of the image header in Pesto to avoid the
"pop" at the end of the animation by laying out the image header at its
final visual size instead of relying on occlusion to size the image
header.

Fixes #5202
Fixes #5204