[RFR] Add read-only mode

A read-only entity doesn't allow access to the mutation views (editionView, creationView, deletionView). In addition, all links to the editionView are replaced by links to the showView.

```js
var tag = new Entity('tags').readOnly();
```
I had to change that because a detail link can link to the show view for read only entities. To avoid breaking BC, `isDetailLink()` still exists but logs a deprecation warning.