Add visual testing

[ceddlyburge]

When converting to elm 0.19, at one point all the colours were white. This passed all the tests, but the screen looked blank and was completely unusable. This never got released, but is a hole in the tests nonetheless.

Visual testing would catch this (it basically compares pictures of how the website looks at various points, and flags if there are changes.

There are plug ins for cypress that allow visual testing, and it doesn't look like it would be very hard to set it up.