Display links to deleted ports differently from links to non-deleted ports

[ryandesign]

Similar to #283, links to deleted ports should be displayed in a visually distinct style (for example grayed out and/or strikethrough) when referenced in other ports.

As an example of how this would have helped me, py37-botocore was just deleted. I looked at https://ports.macports.org/port/py37-botocore/details/ and saw several ports in the "Ports that depend on 'py37-botocore'" section, but I did not realize that all but one of those ports had themselves already been deleted.