FEATURE: Allow linking an existing account during external-auth signup

When a user signs up via an external auth method, a new link is added to the signup modal which allows them to connect an existing Discourse account. This will only happen if:

- There is at least 1 other auth method available

and

- The current auth method permits users to disconnect/reconnect their accounts themselves

spec/requests/omniauth_callbacks_controller_spec.rb

@@ -246,6 +246,19 @@ RSpec.describe Users::OmniauthCallbacksController do
         expect(data["destination_url"]).to eq(destination_url)
       end
 
+      it 'should return an associate url when multiple login methods are enabled' do
+        get "/auth/google_oauth2/callback.json"
+        expect(response.status).to eq(302)
+
+        data = JSON.parse(cookies[:authentication_data])
+        expect(data["associate_url"]).to start_with('/associate/')
+
+        SiteSetting.enable_local_logins = false
+        get "/auth/google_oauth2/callback.json"
+        data = JSON.parse(cookies[:authentication_data])
+        expect(data["associate_url"]).to eq(nil)
+      end
+
       describe 'when site is invite_only' do
         before do
           SiteSetting.invite_only = true