CommentNode: Change icon from orange to grey

We have no other bright solid colours in Apex or Vector so it looks quite
out of place. Also it conflicts with the selection colour in FF/Linux.

Also swapped implementation to use the file from OOjs UI instead since
they're now equal. Made sure dimeneions and alignment stay the same. Couldn't
append an instance of OO.ui.IndicatorElement as it doesn't allow invisible
content (the em space for selection), using the underlying mixin instead.

Also removed top/bottom margin around the inline block which
previously caused the line height of the containing paragraph to
increase by 0.15em in both directions.

Change-Id: Ic2133b9a588050f5fc159cadf0e6f315ae701a10
Merge "CommentNode: Change icon from orange to grey"