Added PostFeedLayout and PostCategoryLayout

[smnh]

Two new models and matching components:

• PostFeedLayout
• PostCategoryLayout

PostCategoryLayout extends the PostFeedLayout, both in model and in component. They are exactly the same. The difference is their urlPath that is configured in stackbit.yaml in nextjs-starter.

Related PRs:
stackbit/stackbit-nextjs-starter#21
stackbit/stackbit-content-packs#15

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[JugglerX]

@smnh I tested the pagination in Studio and it worked. Nice!

• I tested changing the "Number of Posts per page" to 3 (of 4) and pagination just worked.
• I tested adding new post pages and they worked.

[JugglerX]

@smnh @TomasBankauskas couple of missing pieces?

• Do we need to add the category links to the posts?
• I noticed the PostLayout does not display the image @TomasBankauskas

Also see my comment in the content pack PR about category pages being empty and some other issues stackbit/stackbit-content-packs#15 (comment) @smnh

[smnh]

• Do we need to add the category links to the posts?

You mean from UI perspective, yes. Any post that has a category, should show a link to that category.
How links work?
@JugglerX @TomasBankauskas, right now we have a utility function in components that resolves object file path to page path. No more, with this PR in nextjs-starter, every page will have __metadata.urlPath. So no need to call getPageUrlPath anymore.
For example, a blog post that shows category simply uses its __metadata to render the link:

<Link href={category.__metadata.pageUrl}>{category.title}</Link>