[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.
The main reason I went with going on the return value rather than the block was because it'll lead to us having to do an explicit return.  Something like so:

```
  def create
    Core::FeedbackWriter.new(new_feedback).call do
      render :new
      return
    end

    redirect_to :back, notice: 'Your feedback has been submitted'
  end
```