Improve link summary screen to show other linked and unlinked recipes for the same item

[Dorus]

Currently the link summary screen only shows the current link. However YaFC allows the same item to be linked multiple times at different levels or even nested levels.

To give an example, a page might look like this:

• A B -> C [link B1]
  • A -> B [link B1]
• A C -> D [link B1]
  • A E -> F [link B2]
  • A -> B [link B2]

Here the first 3 recipes are all linked to the link B1, but at the nested level of the 3th recipe there is a second link (B2) that hides 2 recipes from link B1.

Currently the link summary will only show recipe 1, 2 and 3 in the above example for link B1, and recipes 4 and 5 for link B2. With this change, the link summary page for B1 will show 1, 2 and 3 as primary recipes and 4 and 5 as related links. Similar with link B1 is removed, the link summary page for B2 will show recipe 1,2 and 3 as unlinked.

I've started working on this in #413

Possibly future improvements:

• Show if related link is a child link, parent link, or nephew link.
• Add buttons to create or remove links on nested or parent recipes.
• Add buttons to create links on unlinked recipes.
• Add buttons to promote existing links to their parents so unlinked recipes become part of them.

I need to think of a proper way to display the relations between recipes on the link summary page.