minimal support for action=channels&method=tree

[martymcguire]

Partially fulfills https://github.com/indieweb/microsub/issues/44

When method=tree is supplied to an action=channels GET request, a sources array will be included with each channel of the form:

{
  "channels": [
    {
      "sources": [
        {
          "_id": 123,
          "url": "http://example.com/",
          "name": "Channel name"
        }
      ]
    }
  ]
}

I haven't included info like per-source unread count or last-checked time. I basically only included "easy" information that was obvious in the DB schema.

No time pressure on this. Feel free to treat it as a discussion piece. :}