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

![filter_journalists](https://cloud.githubusercontent.com/assets/4560985/5184473/bbc201fa-7485-11e4-9df5-ae52d6aa41e7.gif)

This whole method can be `topics.pluck(:id)`. It's more efficient (one SQL query instead of many) and easier to read.