fix: null error at useRouteMatch when running on tutor

tutor sets the PUBLIC_PATH to '/discussions' which causes frontend-platform to
treat all URLs for matching etc to be relative to this path. Since many places
include '/discussions' in the match it causes those matches to break.

This change makes the default PUBLIC_PATH in .env.development to match the one
set by tutor and removes it from the base path of the router letting frontend
platform handle the prefix.

This also allows for deployments to customise this path to be something other
then 'discussions'.

.env.development

@@ -23,3 +23,4 @@ USER_INFO_COOKIE_NAME='edx-user-info'
 SUPPORT_URL='https://support.edx.org'
 LEARNER_FEEDBACK_URL=''
 STAFF_FEEDBACK_URL=''
+PUBLIC_PATH='/discussions'

src/data/constants.js

@@ -1,6 +1,9 @@
 import { getConfig } from '@edx/frontend-platform';
 
 export const getApiBaseUrl = () => getConfig().LMS_BASE_URL;
+export const getFullUrl = (path) => (
+  new URL(`${getConfig().PUBLIC_PATH.replace(/\/$/, '')}/${path}`, window.location.origin).href
+);
 
 /**
  * Enum for thread types.
@@ -137,7 +140,7 @@ export const DiscussionProvider = {
   OPEN_EDX: 'openedx',
 };
 
-const BASE_PATH = `${getConfig().PUBLIC_PATH}:courseId`;
+const BASE_PATH = '/:courseId';
 
 export const Routes = {
   DISCUSSIONS: {

src/discussions/posts/post/Post.jsx

@@ -11,7 +11,7 @@ import { useIntl } from '@edx/frontend-platform/i18n';
 import { Hyperlink, useToggle } from '@edx/paragon';
 
 import HTMLLoader from '../../../components/HTMLLoader';
-import { ContentActions } from '../../../data/constants';
+import { ContentActions, getFullUrl } from '../../../data/constants';
 import { selectorForUnitSubsection, selectTopicContext } from '../../../data/selectors';
 import { AlertBanner, Confirmation } from '../../common';
 import { DiscussionContext } from '../../common/context';
@@ -78,8 +78,7 @@ const Post = ({ handleAddResponseButton }) => {
   }, [closed, postId, reasonCodesEnabled, showClosePostModal]);
 
   const handlePostCopyLink = useCallback(() => {
-    const postURL = new URL(`${getConfig().PUBLIC_PATH}${courseId}/posts/${postId}`, window.location.origin);
-    navigator.clipboard.writeText(postURL.href);
+    navigator.clipboard.writeText(getFullUrl(`${courseId}/posts/${postId}`));
   }, [window.location.origin, postId, courseId]);
 
   const handlePostPin = useCallback(() => dispatch(