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
Would like to extract both of these lines and no need for journalists to be an ivar:

``` ruby
def create
  announcement = load_announcement_from_url # no need for ivars since you only ever redirect away
  create_pitch_for_each(announcement, journalist_ids)
  redirect_to announcement
end
```