Proposal: channel method to include all sources

Nobody has claimed this yet.

Request: ?action=channels&method=tree

The response includes all sources in a channel, order is defined by the server.

Response:

```
{
"channels": [
{
"uid": "31eccfe322d6c48c50dea2c84efc74ff",
"name": "IndieWeb",
"unread": 9,
"sources": [
{
"uid": "sourceId1",
"url": "https://source.url",
"name": "Source name",
"unread": 3,
"last_update": "2020-05-11T17:00:00",
"total": 23,
},
{
"uid": "sourceId2",
"url": "https://source-2.url",
"name": "Source name 2",
"unread": 6,
"last_update": "2020-05-11T17:00:00"
"total": 12,
}
],
}
]
}
```

Use cases:

- 'reader' mode, view the sources in a channel immediately so you can click on them.
- 'admin' mode: ability to have an overview of all channels and sources in one screen instead of having to click individually per channel.

This came out of suggestions of people who are currently testing the Indigenous desktop application and are coming from Feedly. As far as they are concerned, that is a very handy feature (yet, not blocking). Example screenshot at https://s5.feedly.com/images/fx/landing/source-browser-empty-v2@2x.png

Properties of sources:

- url: url of the source/feed
- name: name of the source/feed
- unread: unread items per source (using the same setting as the channel)
- last_update (optional): this would indicate the last time an item was fetched from the source. This is sometimes interesting to know to figure out if a feed is still alive, or possibly even offline.
- total (optional): number of items in the source

Last update and total can be calculated in the Drupal Microsub server.

Start by reviewing the existing `?action=channels&method=tree` response and the Microsub specification to determine how channel data is currently represented. Define the response shape for source URL, name, unread count, and optional last-update and total fields, then verify that it supports the reader and admin use cases described.

Written by the indexing model from the issue text.