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/constants.js

@@ -1,6 +1,6 @@
 import { getConfig } from '@edx/frontend-platform';
 
-export const API_BASE_URL = getConfig().LMS_BASE_URL;
+export const getApiBaseUrl = () => getConfig().LMS_BASE_URL;
 
 /**
  * Enum for thread types.