fix: use getConfig in order to support runtime configuration (#286)

Before, gradebook was reading config from `process.env`
directly, which locked the app into using only
static (build-time) configuration. In order to
enable dynamic (runtime) configuration, we update
gradebook to use frontend-platform's standard
configuration interface: `mergeConfig()` and `getConfig()`.

Bumps version from 1.5.0 to 1.6.0.
(I would normally just do a patch release for a fix, but the version
 was hasn't been bumped for a while, so adding in full runtime
 configuration support seemed like it warranted a proper
 minor version bump.)

Co-authored-by: Ghassan Maslamani <ghassan.maslamani@gmail.com>

src/data/services/lms/urls.js

@@ -1,9 +1,9 @@
+import { getConfig } from '@edx/frontend-platform';
 import { StrictDict } from 'utils';
-import { configuration } from 'config';
 import { historyRecordLimit } from './constants';
 import { filterQuery, stringifyUrl } from './utils';
 
-const baseUrl = `${configuration.LMS_BASE_URL}`;
+const baseUrl = `${getConfig().LMS_BASE_URL}`;
 
 const courseId = window.location.pathname.split('/').filter(Boolean).pop() || '';