Added sonata FrontendLinkExtension to render frontend route button
Optimization of the frontend link button in SonataAdmin

- Added translation
- Shortened caption and added title
- Routes can now also be linked to the frontend
- Improved exception handling
Made it with the translations (google translator), so someone native speaking should really review it.
Merge pull request #273 from frne/frontend-link-sonata-admin

[AdminExtension] Add link to page in frontend if possible
Added css rules for button styling
Merge pull request #274 from frne/sonata-frontend-link-styling

[Fix] CSS classes for the SonataFrontendLinkExtension