feat: Support runtime configuration (second attempt)

(This reintroduces the change in 9f84230c that was later reverted by
67b0b33a.)

frontend-platform supports runtime configuration since 2.5.0 (see the PR
that introduced it[1], but it requires MFE cooperation.  This implements
just that: by avoiding making configuration values constant, it should
now be possible to change them after initialization.

Almost all changes here relate to the `LMS_BASE_URL` setting, which in
most places was treated as a constant.

[1] https://github.com/openedx/frontend-platform/pull/335

src/data/api.js

@@ -1,9 +1,9 @@
 /* eslint-disable import/prefer-default-export */
 import { getAuthenticatedHttpClient } from '@edx/frontend-platform/auth';
 
-import { API_BASE_URL } from './constants';
+import { getApiBaseUrl } from './constants';
 
-export const blocksAPIURL = `${API_BASE_URL}/api/courses/v1/blocks/`;
+export const getBlocksAPIURL = () => `${getApiBaseUrl()}/api/courses/v1/blocks/`;
 export async function getCourseBlocks(courseId, username) {
   const params = {
     course_id: courseId,
@@ -14,6 +14,6 @@ export async function getCourseBlocks(courseId, username) {
     student_view_data: 'discussion',
   };
   const { data } = await getAuthenticatedHttpClient()
-    .get(blocksAPIURL, { params });
+    .get(getBlocksAPIURL(), { params });
   return data;
 }