feat: add a new course goal unsubscribe landing page (#612)

URL format: /goal-unsubscribe/<uuid-token>

This is designed to be used in the new course goals feature, where
emails will be sent to learners and those emails will include a
link to this landing page, as an unsubscribe link.

Also, update calls to the LMS course home API to not include the
/v1/ fragment anymore, as we're moving to an unversioned API.

AA-907

src/setupTest.js

@@ -73,6 +73,7 @@ export const authenticatedUser = {
 
 export function initializeMockApp() {
   mergeConfig({
+    CONTACT_URL: process.env.CONTACT_URL || null,
     INSIGHTS_BASE_URL: process.env.INSIGHTS_BASE_URL || null,
     STUDIO_BASE_URL: process.env.STUDIO_BASE_URL || null,
     TWITTER_URL: process.env.TWITTER_URL || null,