Users can filter journalists by topic

When creating a new pitch, users must select a topic or set of topics applicable to the pitch. Selecting topics will render a list of journalists who cover that topic. A PitchTopic, (a separate record of a pitch's topics) is also created when a pitch is created. PitchTopics will be used to allow for further pitch sorting.

Trello: https://trello.com/c/cDVqB4IE/48-filter-journalists
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