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>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@edx/frontend-app-gradebook",
-  "version": "1.5.0",
+  "version": "1.6.0",
   "description": "edx editable gradebook-ui to manipulate grade overrides on subsections",
   "repository": {
     "type": "git",