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)

Oh, because `.map` is not a jQuery function, it's a built-in JS function (that works only with newer browsers). This is fine as long as you don't care about IE (which I wouldn't, for this).