Add gray section below merged, merging, and auto-merge states of MR merge widget

The following discussion from !71721 (merged) should be addressed:

• @pedroms started a discussion: (+2 comments)

  @iamphill ❓ question (style,non-blocking): we're missing this new “gray section” style for the “Merged” state. Do we have an issue for it?

  This MR	Expected