[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.
You could do the following for aesthetics:

```rb
def create
  Core::FeedbackWriter.new(new_feedback).call do
    render :new and return
  end

  redirect_to :back, notice: 'Your feedback has been submitted'
end
```