Add flicker view to review page

[lencioni]

The concept here is similar to stereoscopic gifs, like this one:

But instead of creating the illusion of depth, I think this will make some kinds of differences easier to spot and understand.

I think there are some cases where it might be useful to have a view that switches between the before and after screenshots rapidly.

We could maybe make it so you can click to stop it, and then click to toggle between before and after, and double-click to resume the flickering (or something like that).

[trotzig]

Can we make all diffs look like ^^? :)

[lencioni]

It might actually make sense to combine this with #138 since they are very similar in nature.