Posts: Improve PageIndex datasource, pagination. Add 'post' pagetype and support published_date field

[sam-shift72]

Improvements to support blog post pages.

• Add post page type and support a published_date on pages.
• PageIndex data source: configurable filter and sort to enable a blog posts in reverse chronological order paginated listing
• Refactored pagination. Allow configuring a different path for the first page, e.g. /posts/ then /posts/2/, /posts/3/ etc. for subsequent pagination.
• Page data source: warn instead of failing site build if the template doesn't support a particular page type

Extra Datasource Configuration

You can pass extra options through to the datasource in your Kibble.json, for example:

{
  "routes": [
    {
      "name": "postIndex",
      "datasource": "PageIndex",
      "urlPath": "/posts/:index/",
      "options": {
        "pageTypes": ["post"],
        "sortBy": ["published_date:desc", "title:asc"]
      },
      "pageSize": 10
    },
  ]
}

The PageIndex source supports filtering down to particular page types to include in the index listing, as well as sorting criteria.

Pagination

You can configure a different path for page 1 of paginated index by using firstPageUrlPath

{
  "routes": [
    {
      "name": "pageIndex",
      "datasource": "PageIndex",
      "firstPageUrlPath": "/pages/",
      "urlPath": "/pages/:index/",
      "pageSize": 10
    }
  ]
}

Internally, pagination has been refactored so that it's now a standalone concern that can easily be added to any data source that needs it by calling ctx.Paginate(len(items)) on the render context. This returns a list of pages that can be iterated over, and calculates the slice indexes for each page.

Caveats

Currently, configuring routes for different kinds of pages isn't supported.

I had initially added the pageTypes filter to the Page datasource, so that you could define different routes to render specific types of pages — blogs on /posts/, curated pages on /curated/ and everything on /pages/

{
  "routes": [
    {
      "name": "postItem",
      "datasource": "Page"
      "urlPath": "/posts/:slug",
      "options": {
        "pageTypes": ["post"]
      },
    },
    {
      "name": "curatedPageItem",
      "datasource": "Page"
      "urlPath": "/curated/:slug",
      "options": {
        "pageTypes": ["curated"]
      },
    },
    {
      "name": "pageItem",
      "datasource": "Page"
      "urlPath": "/page/:slug"
    },
  ]
}

While this generated the site correctly, it exposed an issue with the way Kibble's link generation works: the routeTo functions will just pick the first route whose datasource claims to handle a particular entity/slug. With the above config, every link would be generated as /posts/....

Supporting this will take a bit more refactoring, such as instantiating each datasource per-route so that methods like IsSlugMatch, GetRouteForSlug, and GetRouteForEntity can have access to the datasource options