This change adds keyboard shortcuts to the "gr-diff-comment-thread"
expand all comments when 'e' is pressed and collapse all comments when
'shift + e' is pressed. Note that the keyboard event is detected on the
thread instead of the comment to minimize the number of events getting
triggered.
Feature: Issue 4738
Change-Id: Iab77349bd1527d7af5e05a827919a78a86909835