Added warning if javascript is not enabled

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT

We have a lot of javascript functionality in SonataAdmin. This PR adds a warning for users, that don't have the javascript enabled.

![js_warning](https://cloud.githubusercontent.com/assets/960844/4176626/c2208438-360d-11e4-847b-c769fcfac455.jpg)


Thank you for review @rande. I've changed it to `Javascript is disabled in your web browser. Some features will not work properly.`.