Comment thread view

Recursive view that groups a comment and all of its replies, which visualizes the discussion tree starting from a particular root comment.

## Depends on

#11, #47.

## Constructor options

name | description
---|---
`model` | An `oa:Annotation`, ideally with an `oa:bodyValue` or that `oa:hasBody`. This is the root of the discussion tree that will be visualized.

## Rendering

At the top, a comment view (see #47) with the same `.model`. Below that, a slightly indented vertical list of comment thread views (this issue), one for each `oa:Annotation` with a body that `oa:hasTarget` the `.model`.

If the `.model` has no body of its own and the current user has permission to do so, we *might* want to show just a comment submission form like in #47 so that a comment can be added after the fact. In this case, the comment form should be a view in its own right.

## Special methods

None required yet. Might want to add a `.collapse()` method in a later issue.

## Events

name | description
---|---
`reply` (annotation, reply) | Propagated event from embedded comment views.