Removing old references to populate_repo_browser

Removed references to populate_repo_browser in controller, tests and routes config; copied submissions_controller.rb to lib/benchmarking/submission_files/ to update it.
@david-yz-liu, going by https://www.relishapp.com/womply/ruby-style-guide/docs/exceptions, should I change these parts to something like:

      begin
        revision = repo.get_revision(rev)
        unless revision.path_exists?(
            File.join(@assignment.repository_folder, @path))
          fail 'path does not exist at revision'
        end
      rescue => error
        revision = nil
        raise if error.message != 'path does not exist at revision'
      end

?