GitHub issue sidebar tweak

Another tweak I pushed out was to once again surface notification reasons in the sidebar. Prior to the timeline redesign, we had the reason clearly visible at all times at the bottom of the issue. With the redesign, we moved it to a tooltip in the sidebar.

Now, the text is always visible and a bit shorter now. I thought I'd need this text less, but it's definitely come in handy as a person who always uses GitHub's web notifications.