Migrate away from `ember-data` and towards a custom API implementation

[spuxx-dev]

ember-data is an extremely powerful framework for fetching and storing data. However, due to the nature of the backend systems that potber communicates with and the nature of the app itself, it makes little use of ember-data's strong points (e.g. caching, optimistic updates and more).

Furthermore, ember-data is currently in the process of a change in paradigm. The old concept of models, serializers and adapters is being replaced by a new pattern of request managers and less abstraction. While this is certainly a positive development, migrating to the new pattern will increase the work load of maintaining and developing potber. As a result, potber will be migrating off of ember-data and instead implement a much simpler custom data fetching service.