Fine-tune vertical scroll behaviour on the translations page

[henrycatalinismith]

In #107 we've laid down the broad strokes of the translations page. One of the finer details that could do with revisiting is the vertical scroll behaviour of the MessageList component. Currently, the body tag has overflow: hidden on this page, with the vertical scroll on the MessageList instead. This might be nicer if we can figure out how to reinstate the normal body scroll while preserving the other aspects of the layout. Then normal things like pressing space to scroll the page will work. It may also help with an issue where the last MessageForm is partially cut off on mobile (see #107 (comment)).

[henrycatalinismith]

Hmm not off to a great start.
bvaughn/react-window#192

What your'e describing isn't supported by react-window. It was supported by react-virtualized, via a WindowScroller component, but WindowScroller was one of those painful to maintain components.