Add an epics board index page

Although epics board will use our GraphQL API for working with epic boards/epic lists, we still need a "landing page" which will take care of initializing FE components - for issue boards this is handled by app/controllers/boards/lists_controller.rb's index action - we will need similar endpoint for epic boards too which:

• automatically creates default epic board if there is not any board yet
  • on BE side autocreation logic is part of app/services/boards/lists/list_service.rb - we can extract this logic into a concern shared both by issue and epic boards
  • also then this service should be used to list epic board lists in graphql resolver - !49728 (comment 467780323)
• renders epic board html page with the code to initialize JS components
• (can be done separately) expose this epic board's url in the GraphQL API same as we will expose web_url for issue boards (related to #282299 (closed))