As reported [on Meta](https://meta.discourse.org/t/discourse-subscriptions/140818/352?u=justin), moderators could access all of the subscriptions data (plugins/prices/subscribers) and manage them. This should not be the case, so this PR adds a route constraint to 404 moderators from these routes.
Adds full support to create coupon/promo codes in the Admin > Plugins > Subscriptions section of the plugin. The Create Coupon button opens a form on the same page, and the active checkboxes toggle the active status of the coupon code.
Loïc Guitaut
Jarek Radosz
David Taylor
Justin DiRose