Episode-entity relationship form

[lukavdplas]

This adds forms for:

• Editing related episodes on the agent form
• Editing related agents, locations, letters, and gifts on the episode form

Close #125

Includes:

• Some mutations and a query to handle the relationship between an episode and a related entity (i.e. an agent/location/gift/letter). Also adds some interfaces to work with shared properties.
• GraphQL types are now explicit that name and description are non-nullable in the query output.
• A EpisodeEntitiesComponent to be used in the episode form for agents, locations, gifts, and letters.
• An AgentEpisodesComponent to be used in the agent form. This one is not directly reusable (yet).
• An EpisodeLinkComponent and CollapsibleCardComponent as more basic building blocks.

Not (yet) included:

• You can't yet edit related episodes on the forms for locations, letters, and gifts. This was harder to generalise than the other way around, but could still be done in the future. Or we could just copy the AgentEpisodesComponent.
• The dropdown to add a new item includes a "create" button, but it's not yet implemented.

[XanderVertegaal]

"entity_type" does not exist on the model, so it does not belong here.

[XanderVertegaal]

Very impressive how you managed to implement this despite the sparsity of the documentation. Well done!

I have a lot of nit-picky comments, but functionally, the code works great! Consider it approved after you've covered my remarks.

[XanderVertegaal]

Suggested change

	return EpisodeAgent.objects.filter(agent=parent)
	return EpisodeAgentType.get_queryset(EpisodeAgent.objects, info).filter(agent=parent)

[XanderVertegaal]

Suggested change

	return EpisodeLetter.objects.filter(letter=parent)
	return EpisodeLetterType.get_queryset(EpisodeLetter.objects, info).filter(letter=parent)

[XanderVertegaal]

Suggested change

	return this.linkTo == 'episode' ? 'episode' : this.entityName;
	return this.linkTo == 'episode' ?? this.entityName;

(All of these getters would be sweet computed()s by the way, but that's something to consider when we're less strapped for time.)

[XanderVertegaal]

private? Also a better name than query would be helpful.

[XanderVertegaal]

This object is very similar to entityTypeNames in utils.ts. For consistency's sake, please put this object there as well. (You can maybe merge them as well, or derive one from the other). If you've done that, you can replace this function's body with return linkTo == 'episode' ? 'episodes' : entityTypeRoutes[entityType];