Allow independent scrolling of two files

[fulaphex]

In some cases, the diff algorithm doesn't work well and doesn't align files well. One cannot then see the relevant sections of the files together and compare them easily without constant back and forth scrolling. The solution I have in mind is a toggle that would allow independent scrolling of reviewed files.

[smacker]

JFI: because we sync both files using JS it should be super easy to implement.