Add support for abandoning a dead change Add the ability to abandon a change. When the abandon button is clicked, a dialog will pop up for the user to enter an abandon message. This message will be added as a comment to the change and an email will be sent to all reviewers of the change. Bug: GERRIT-41 Signed-off-by: Brad Larson <bklarson@gmail.com> Signed-off-by: Shawn O. Pearce <sop@google.com>