Add labels for "Before" and "After" in happo diff viewer.

[mereskin-zz]

It has a title but I feel like it's not discoverable enough. Explicit labels would ease the struggle.

[lencioni]

We had something a little more discoverable in 7d7bab0 but it was removed in 0264d8b. I agree this is an opportunity to improve the interface, but I'm not entirely sure what the best solution would be.

Perhaps we should tackle this at the same time as #206.