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/discussions/data/hooks.test.jsx

@@ -10,11 +10,12 @@ import { AppProvider } from '@edx/frontend-platform/react';
 import { initializeStore } from '../../store';
 import { executeThunk } from '../../test-utils';
 import { DiscussionContext } from '../common/context';
-import { courseConfigApiUrl } from './api';
+import { getCourseConfigApiUrl } from './api';
 import { useCurrentDiscussionTopic, useUserCanAddThreadInBlackoutDate } from './hooks';
 import { fetchCourseConfig } from './thunks';
 
 const courseId = 'course-v1:edX+TestX+Test_Course';
+const courseConfigApiUrl = getCourseConfigApiUrl();
 let store;
 let axiosMock;