Animate snapshot thumbnail while still processing

Before a snapshot has been taken, we render a clock icon and animate its
opacity up and down to signify that it is actively being worked on and
is not ready to be reviewed. We should keep animating these after the
snapshot has been taken, before the diff has been generated to clearly
communicate that these items are not ready for review yet.

Closes #101.

Change-Id: I5118b9a87d0d7ae03b56383b7298400fe87bfa78
Remove unused .snapshot-baseline style

We haven't used this style since f279eff0d1ac9362285.

Change-Id: I3b03a5d733d6b3b150267ff936759c138a5d3e91