Use Symfony Panther for functional tests

[TomaszGasior]

Current functional test use Symfony BrowserKit which just boostraps Symfony framework and fakes HTTP requests. Most of the time BrowserKit based tests are fine but they don't verify whether frontend specific HTML, CSS and JavaScript work correctly. For example, broken HTML validation may make creating new radio stations impossible but current BrowserKit-based tests won't show that.

It's needed to verify in which tests it may make sense to use Panther instead of BrowserKit.