Switch from single var to one-var-per-variable

Frontenders at Airbnb at have been discussing this change for a bit, and
we've come to favor one-var-per-variable over one-var-only declarations.
Two things improve the maintainability of this style over one var for
multiple variables:

 1. You never have a diff of a line that's removing a `;` and inserting
    a `,`.
 2. You can't accidentally declare global variables because you have a
    one-character mistake (semicolon instead of comma):

    ```javascript
    var foo = 1,
        bar = 2;
        baz = 3; // added later and (accidentally) declared globally
    ```