[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.
Checking ``params`` should be in ``feedback_params``:

```ruby
def new_feedback
  @new_feedback ||= Core::Feedback.new(feedback_params)
end

def feedback_params
  if params[:feedback]
    params.require(:feedback).permit(:type, :subject, :body)
  else
    {}
  end
end
```