[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();
```
[Done](https://github.com/marmelab/ng-admin/commit/0db067a91737a5acfca851ac91bcea9f3dbeb1d6)