[Fix] CSS classes for the SonataFrontendLinkExtension

Had to add specific CSS classes to identify the button for styling in symfony-cmf/cmf-sandbox#276.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT
| Doc PR        |