Motivation: aligning us with JS/Ember practices (runtime deps in `dependencies`, build/dev-time deps in `devDependencies`)
1. Move deps to devDeps where applicable (rule of thumb: it's a devDep unless it's required at runtime by the rails app or it's imported in the addon's code)
2. Remove unused dependencies and add missing ones (in addons)
3. Remove empty `repository` fields
4. Move `engines` and `ember` fields to the bottom