Add an icon for comment issue verification.
Review Request #9095 - Created July 20, 2017 and submitted
This icon is used to represent an issue that is pending verification by
the reviewer. It has the same shape as an open issue, with a more
subdued (but still present) color, and arrows pointing in opposite
directions to indicate a back-and-forth between reviewer and developer.
.rb-icon-issue-verifyingCSS class has been added for using this icon.
Tested in the browser by replacing the icon's CSS class on an open issue.