move discount-codes plugin to its own npm package, and install npm version Reaction

[kieckhafer]

New repo: https://github.com/reactioncommerce/plugin-discount-codes

Convert the discount-codes plugin. This includes moving all related files into a new repository, releasing that repo as an npm package, and then updating this repo, reaction, to use the npm version of the plugin.

Part of this epic to move all plugins into their own repositories and release as npm packages.