Use on modifier for expand toggle

[kpfefferle]

• I pulled in ember-decorators as the {{on modifier wasn't working without the @action decorator
• Installing ember-decorators tries to upgrade @ember-decorators/babel-transforms to ^3.1, but that breaks the build
• Reverting ``@ember-decorators/babel-transformsto^2.1.2` gets everything working, though with a warning
• Edit: I used a hack suggested by @mixonic to load the @export decorator from @ember/object until fix export -- current error: @ember/object does not have a action export emberjs/ember.js#17727 is resolved
• Hopefully we'll get the native @action decorator unblocked so we can drop ember-decorators and it won't be an issue

This is a more obvious use of Element Modifiers for #32

[rwjblue]

btw, I'm working on @action export stuff this afternoon