more refactoring

app/models/project.rb

...e/20160302152808_remove_wrong_import_url_from_projects.rb

db/schema.rb

lib/gitlab/github_import/importer.rb

lib/gitlab/github_import/wiki_formatter.rb

lib/gitlab/import_url.rb

lib/gitlab/import_url_exposer.rb

spec/lib/gitlab/github_import/project_creator_spec.rb

spec/lib/gitlab/github_import/wiki_formatter_spec.rb

spec/lib/gitlab/import_url_exposer_spec.rb

spec/lib/gitlab/import_url_spec.rb