Add keyboard navigation to review

This commit adds left/right key navigation to move between snapshots in
the "In review" panel on the snapshot page. We might want to switch to
"[" and "]" instead of left and right to stay more in line with e.g.
Gerrit code review. But for now left and right seem like the right
start.
Fix bug in `snapshot#waiting_for_diff?`

We didn't check if the baseline was the snapshot itself. This led to
accepted snapshots who were first in the set of snapshots for an Url
were shown as "Pending".