Enable GH and BB Server importer UI when override flags are enabled

What does this MR do and why?

Similar to Enable Direct Transfer UI when override flag is... (!163481 - merged) for Direct Transfer.

Currently, when GitHub or BitBucket Server Importer are disabled in the application settings, one can still use the API by enabling one of the override feature flags override_github_disabled or override_bitbucket_server_disabled FF. However, the UI will remain disabled.

With this change, users can use these two importers via both the API and the UI when the corresponding override FF is enabled for a given user.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

spec/controllers/import/bitbucket_server_controller_spec.rb

@@ -18,7 +18,7 @@ def assign_session_tokens
 
   before do
     sign_in(user)
-    allow(controller).to receive(:bitbucket_server_import_enabled?).and_return(true)
+    stub_application_setting(import_sources: ['bitbucket_server'])
   end
 
   describe 'GET new' do
@@ -69,6 +69,35 @@ def assign_session_tokens
       end
     end
 
+    context 'when bitbucket server importer is not enabled' do
+      before do
+        stub_application_setting(import_sources: [])
+        stub_feature_flags(override_bitbucket_server_disabled: false)
+      end
+
+      it 'returns 404' do
+        post :create, params: params, format: :json
+
+        expect(response).to have_gitlab_http_status(:not_found)
+      end
+
+      context 'when the override_bitbucket_server_disabled flag is enabled' do
+        before do
+          stub_feature_flags(override_bitbucket_server_disabled: true)
+        end
+
+        it 'returns 200' do
+          allow_next_instance_of(Gitlab::BitbucketServerImport::ProjectCreator) do |service|
+            allow(service).to receive(:execute).and_return(project)
+          end
+
+          post :create, params: params, format: :json
+
+          expect(response).to have_gitlab_http_status(:ok)
+        end
+      end
+    end
+
     it 'returns an error when an invalid project key is used' do
       post :create, params: { repo_id: 'some&project/repo' }