3801 delete own photos

Comments welcome.
What about this:

In the User model:
```js
  isAuthorOf: function(post) {
    return this.authenticated() && post.get("author").id == this.id;
  }
```

And then in the views presenter overrides:
```js
authorIsCurrentUser: app.currentUser.isAuthorOf(this.model)
```

A bit more verbose but the logic is encapsulated in a single place.