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.
Unused method argument - `params`. If it's necessary, use `_` or `_params` as an argument name to indicate that it won't be used. You can also write as `create_multiple_pitches(*)` if you want the method to accept any arguments but don't care about them.