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>
