This patch adds keyboard shortcuts to expand & collapse the diff in a review

request and in a diff fragment in a review. Expand is triggered by the e 

key and collapse is triggered by the w key (the c key may seem more 

natural, but it's taken already by another shortcut so w was chosen for its 

proximity to the e key for ease of use between them).
For the diff viewer, expand and collapse buttons are now tracked as anchors

in RB.DiffViewerPageView. This allows us to find the next button on the 

page from the currently selected anchor and trigger it when a keyboard

shortcut is used. The e and w keys are added to the existing key

bindings mixin object to track those key presses and trigger the right 

button.
For diff fragments, there can only be one set of expand buttons and one 

collapse button in any given fragment. If the fragment is focused, the e

and w keys will trigger their respective expand and collapse buttons. This

is implemented by using the key bindings mixin in RB.DiffFragmentView. The 

shortcut to expand the fragment defaults to expanding the diff by 20 lines,

which is achieved by adding a modifier to desired expand button.

Manual testing & unit tests for the e and w key events in both the diff

fragment and diff viewer.