Move the metadata-ery area in PR to under the comments tab
    
    Previously, we had the metadata area that had the merge
    button, the assignee, and the changes summary as a top
    level col on the PR page. This made the diff also be
    in a narrower column, making reading diffs a harder on
    narrower screens,
    
    This commit changes moves these items under the comments
    tab, leaving a full-width of column space for the diff and
    commits tabs