Login refactor 2

Contains the login refactor and webpack
Events (in json-api-client resources and PFE's generic `Model` class) do not have payloads (by design--the source of data should always be the resource/model itself), so this mixin probably won't be very useful.

There's probably a need for a mixin to handle the `change` event of a Resource (and maybe the `change` event of a Type [which fires in response to its resources `create` and `delete` events]), but I want to wait until some resources other than the current user are in place before deciding which bits can be generalized. I'll be working with Projects real soon, let's revisit this then.