Adds support for the channels extension in Micropub requests. Fixes #773.

This can be used to dictate where a post gets published within a publication, and potentially allows for posts of the same type to be published to different locations.

Adds:

• New publication.channels configuration option
• Querying /micropub?q=config includes a channels property with configured channels
• Querying /micropub?q=channel returns configured channels

If channels are configured:

• The default channel is the first value given for publication.channels
• If a request includes an mp-channel value, it gets saved to properties.channel
• If a request doesn’t include mp-channel, the default channel UID is used
• If a request includes values for mp-channel that don’t exist in publication.channels, they’ll get ignored. If no values match, the default channel UID is used
• Post form shows a ‘Channels’ field under ‘Advanced options’, with checkboxes to select for each of the configured channels
• Post page shows channels specified for a given post
• A {channel} token is available to use in paths; if multiple channels are assigned to a post, these will get separated using the slug separator character

Example

In publication config:

publication: {
  channels: {
    en: {
      name: "English"
    },
    de: {
      name: "German"
    }
  },
  postTypes: {
    article: {
      post: {
        path: "_posts/{channel}/{yyyy}-{MM}-{dd}-{slug}.md",
        url: "{channel}/{yyyy}/{MM}/{slug}"
      }
    }
  }
}

GET /micropub?q=channel

"channels": [
  {
    "uid": "en",
    "name": "English"
  },
  {
    "uid": "de",
    "name": "German"
  }
]

Published to /en/2024/11/channels:

POST /micropub
Content-Type: application/json

{
  "type": "h-entry",
  "properties" {
    "mp-channel": "en",
    "name": "Channels",
    "content": "Hello, world."
  }
}

Published to /de/2024/11/kanale:

POST /micropub
Content-Type: application/json

{
  "type": "h-entry",
  "properties" {
    "mp-channel": "de",
    "name": "Kanäle",
    "content": "Hallo Welt."
  }
}

How should this value be used by presets?

Right now, if a publication has channels configured, the default (or chosen) channel(s) will get saved to properties.channel, so for example a post might look like this:

----
published: 2024-11-16
title: Channels
channel:
  - en
----
Hello, world.

I think channels are analogous to Eleventy collections, so potentially for that preset, the channel property should be used for tags? But you might want to use properties.category for the tags field instead. Not sure what is the right approach, but can figure that out in a separate PR.

Might this value have equivalent representations in other platforms, too?

Should a default channel be enforced?

Right now, a default channel is enforced. That means, while you might only want to use the channel property for the output path, the channel property would still get added to frontmatter properties as well. Maybe that’s fine (though this’d make be twinge…)