refactor(client): Extract form decorators into their own modules.

The goal here is to extract the decorators from `form.js` so they can be used elsewhere (like in the service-mixin [here](https://github.com/mozilla/fxa-content-server/pull/1437#discussion-diff-15283468R87)). I'm not 100% sold on this, but it does remove a lot of extra, non-essential code from `form.js`