Add journalist topics

Users can select topics when creating a journalist.

NOTE: Creating new topics will be covered in separate commit. This feature is currently using pre-defined topics.

Trello: https://trello.com/c/OdlUwErr/47-add-topics
Ah, I had started building pitches on a different branch, then switched to focus on topics first. I must have done something causing the pitches migration to stay within my schema. I can fix.