Re: [web-annotation] Rename `role` to `motive`

The term `purpose` has grown on me. :smiley:

Annotation
- hasMotivation bookmarking
- hasBody (which) hasPurpose tagging

That reads OK. And possibly more clearly (to some at least) than the same
set of (pseudo) statements made with `hasRole`.

Or...less pseudo:

```
{
  "motivation": " bookmarking",
  "body": {
    "purpose": "tagging",
    "text": "W3C Spec"
  }
}
```

:+1: for `purpose` if we want to spend the time here, but let's not spend
too much more...k? :smiley: Bigger fish and all that. :wink: