Users can create pitches for journalists

Trello: https://trello.com/c/0oul2GrI/44-users-create-pitches

In this feature, users can create write one pitch, assign multiple journalists and have an individual pitch created for each journalist.

I suspect the batch_pitch_params part of the batch_pitch controller can be refactored.

Includes addition of pry & better_errors gems
``` ruby
def create
  @announcement = load_announcement_from_url
  # this will require updating the form so params[:journalists] returns the array of ids
  params[:journalists].each do |id|
    current_user.pitches.create(pitch_params.merge(journalist_id: id))
  end
end

...

private

def pitch_params
  ## this will require updating the form so it properly nests these
  params.require(:pitch).permit(:subject, :title)
end
```