Add a marker for relations to deleted pages

In the current version, if page A has a relation to page B and page B gets deleted, on the UI of relations of the page A the relation to page B is displayed in the same way before and after deletion.

Since links to inexistent pages can happen for various reasons (actual delete but also rename of the target, outdated data), it would be interesting to mark the links to inexistent pages so that the editor of page A can know that there is an issue with relations and fix them manually.

Note that it's also envisageable to fix some "broken" relations automatically - see PAGEREL-6.