Error 500 "OAuth configuration for GitHub missing" when clicking "List your GitHub repositories" on the import page

When on the GitHub import page (e.g. https://gitlab.com/import/github/new), if you click the "List your GitHub Repositories" button without entering an access token and OAuth is not configured, you get a 500 error "OAuth configuration for GitHub missing".

github-import-bug-when-no-access-token-given

That's because in app/controllers/import/github_controller.rb#provider_auth we try to authenticate with OAuth if there's no token in the session, without checking first that OAuth is configured.

I think we should do 2 things:

• make the field mandatory on the frontend and show an error if it's blank
• make the field mandatory on the backend and show an error if it's blank and OAuth is not configured