Allow GitHub imports via token even if OAuth2 provider not configured

Previously, the GitHub importer would only work if the site configured
an OAuth2 provider were configured. Users attempting to import via a
GitHub personal access token would see an Error 500 due to a failed
redirection. We fix this by only doing the redirection if the provider
has been configured and allowing users to see the new import page.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/38524

app/controllers/import/github_controller.rb

@@ -7,7 +7,7 @@ class Import::GithubController < Import::BaseController
   rescue_from Octokit::Unauthorized, with: :provider_unauthorized
 
   def new
-    if logged_in_with_provider?
+    if github_import_configured? && logged_in_with_provider?
       go_to_provider_for_permissions
     elsif session[access_token_key]
       redirect_to status_import_url

changelogs/unreleased/sh-fix-github-import-without-oauth2-config.yml

+---
+title: Allow GitHub imports via token even if OAuth2 provider not configured
+merge_request: 23703
+author:
+type: fixed

spec/controllers/import/github_controller_spec.rb

@@ -16,6 +16,15 @@ describe Import::GithubController do
 
       get :new
     end
+
+    it "prompts for an access token if GitHub not configured" do
+      allow(controller).to receive(:github_import_configured?).and_return(false)
+      expect(controller).not_to receive(:go_to_provider_for_permissions)
+
+      get :new
+
+      expect(response).to have_http_status(200)
+    end
   end
 
   describe "GET callback" do