make comment on PR diffs fit the parent, and not overflow horiz
    
    Fixes #963
    
    Sice the PR diffs are displayed by a 3rd party tool, they are currently
    put into a table, and the comment feilds we are appending to them are
    stretched out to the size of the length of the line in the diff, which
    is not a good way to present it
    
    This patch adds some css styles to rescrict the size of these comment
    boxes at the different viewport widths, so it will not scroll
    horizontally.