Allow close reasons to vary by post type

[cellio]

Each community has a set of close reasons that apply to all closable posts. Any top-level post type can be made closable though configuration, but not all reasons make sense for all post types. On the codidact.com network we're discussing making articles closable so they can be marked as duplicates, but it doesn't make sense to close an article as too generic. We would like to be able to indicate which post type(s) a close reason applies to.

Moderators can edit the close reasons. The interface looks like this:

The edit interface for a single close reason looks like this:

The request is to add one more field to the editor, and find a way to display it in the list: "applies to post types". The value should be a multi-selector for post types, perhaps like the one in the category configuration:

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The motivating use case for this request is to allow articles in the Code Golf sandbox to be closed as "finalized" (with a link, like duplicates) and also to have a close reason like "done" for sandbox posts that are no longer being worked on (and weren't posted as challenges). That said, other communities have articles too, and on some communities an "obsolete" close reason could apply to articles but wouldn't make sense for questions. Also, new post types can be created, and we don't know what their closure use cases might look like.

One might ask if we should instead be looking at per-category close reasons. That might be something we also want, but I don't think it would remove the need for this request. Some close reasons are inherently about questions, and some are inherently about articles, and a category can use both post types. Supporting per-category close reasons is also a more complex UI problem, and since it's not sufficient, I'm proposing to focus first on per-post-type close reasons.