[merge third] Move smartdown adapter logic to the gem

Interaction between the smartdown gem and the smart-answers frontend application used to be part of the smart-answers application. This meant that concerns such as deducing whether we were on a question page, coversheet or outcome depending on the presence of specific nodes, or figuring out what the title for each question was from the markdown was done on the frontend side. 
This PR removes the code from the smart-answer application and moves it in the gem. 
DirectoryInput should live outside the gem and implement methods from SmartdownInput object.