v2.1.0

• Dropped PHP < 5.6 support.
• Dropped Symfony < 2.7 support.
• Dropped jQuery < 3 support.
• Dropped HHVM support.
• Added Symfony 4 support.
• Signature of ThreadPermalinkListener has been changed.
• Route ThreadController::getThreadsActions throws 404 if called without id's.
• Added pipeline parser to integrate several parsers together.
• Added flat view for async thread.
• Removed HTTP class constants in ThreadController.
• Fixed form disappearing when creating a new reply.
• Fixed failing validator with encoded permalink.