[WIP] Feedback - basics of implementation

Just putting this out there for some feedback, mainly in terms of the stuff in `lib/core`, what with this being the first time data is coming out of the app rather than just consuming.

Fundamentally there's nothing complicated being done, we're just ferrying a bunch of data across to a mailer, so the repository and interactor have little to them.
Inheriting from ``Entity`` may be the thing to do here as all this model/entity stuff is defined within it. It would mean doing something like this though which is a bit meh:

```ruby
class Feedback < Entity
  def initialize(attributes = {})
    super('feedback', attributes)
  end
end
```