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
`@pitch`