refs https://github.com/TryGhost/Toolbox/issues/308
- I recently pluralised the API endpoint but never made the changes to
the controller file and everywhere else it's needed
- this commit cleans up that inconsistency so it should be clearer
refs https://github.com/TryGhost/Toolbox/issues/308
- we had a few mentions of `subscribers` in the test suite data generator
but this shouldn't be used any more because it's an ancienttttt concept
- removing this for v5 as it helps to clean the codebase
refs https://github.com/TryGhost/Team/issues/1550
- Switched to using the newsletter design settings over the global settings
- Made the `newsletter_id` property available in the Admin API Post resource
- Added the `showHeaderName` variable that can be used in the post html template
refs https://github.com/TryGhost/Toolbox/issues/169
- Before releasing Ghost v5 we would like to move all canary-related URLs to a non-versioned format, which will become a default in v5.
- 'canary' is by definition unstable, so breaking any unprepared client explicitly using the canary is expected
- Removed the aliased /content/ and /admin/ apps from app.js because with updated configuration they become duplicates of 'canary' endpoints
