Channel Order

Having used [Indigenous](https://indieweb.org/Indigenous) and [Together](https://indieweb.org/Together) for a little while, it's quickly become apparent that I want to be able to set an explicit order to the channels.

The Flickr API has a nice mechanism for ordering, which involves very little effort on the part of the client:

"Ordered, comma-delimited list of photo ids. Photos that are not in the list will keep their original order" https://www.flickr.com/services/api/flickr.photosets.reorderPhotos.html

In Microsub, this could be done in a similar way with a "channel" action:

```
POST /microsub

action=channels
&method=order
&channels[]=2c5e5d7c4d57d
&channels[]=c972846e8278
```

This would set the order of the two channels given. All other channels would remain in the same order. This enables a simple drag and drop interface or a "move up/move down" button in clients, while also providing the ability for more complex interfaces that want to set the order of all channels in one command.

The issue names the Microsub endpoint and its channels action, but no repository files or tests. Start by locating the specification section that defines channel actions and review existing ordering behavior. Done means documenting the ordering method, including how omitted channels retain their order, with an example matching the proposed request.

Written by the indexing model from the issue text.