New ways to see links to item

* New frontend view.
* New API endpoint.
Probably this should do something like my gist did, i.e. `Item.query.filter_by(id=item_id).options(db.joinedload('items_linked').joinedload('item')).first()`

This ensures that the item, its reverse links, and their respective items are all loaded in one query. This also means you don't need to do the loops and extra loading below, and can instead just do something like:

```
for item_link in item.items_linked:
    ...
    value=extract_value(item_link.item.map_dict, path)[0][1]
```