core: Custom retrofit converter to remove DataWrapper<T>

[JoaquimLey]

Using customer converters we can delegate the deserialization of the meta-data to the HTTP client, instead of having the abstract model Jake Wharton's talk on custom converters.

Context

This enables us to remove unnecessary boilerplate encapsulating models/code.

Process

See the talk then:

1. Create custom converter
2. Configure the custom converter to deserialize the metadata
3. Feed the actual model(s) directly
4. Make necessary Presenter(s), Service interfaces changes