Indicate that annotation is created by author of content in publisher groups

## Feature Request Form
### Problem you are trying to address with this feature
It would be fantastic if we could configure the client in such a way to indicate the hypothesis username of authors of the content being annotated on. It would also be useful to indicate which hypothesis usernames were recognised as staff of the publisher.

### Your solution
```javascript
window.hypothesisConfig = function () {
return {
usernameIcons: [{
"username1": "https://openclipart.org/download/14693/Rocket000-emblem-star.svg",
"username2": "https://openclipart.org/download/14693/Rocket000-emblem-star.svg"
}],
};
};
```

If the `usernameIcons` array is present then as an annotation card is prepared check if there is an entry for the username and then output the icon to the side of the username.

The issue names no implementation files or tests; start by locating the client configuration entry point for window.hypothesisConfig and the annotation-card rendering path. Check how usernameIcons would map usernames to icons, then verify that configured icons appear beside matching usernames and that publisher staff usernames can be recognised.

Written by the indexing model from the issue text.