Format display name/email pairs in JSON as dicts, not arrays.

Summary:
Not sure if this is what we actually want to go with for the API, but if so
here's an expedient way to do it.

Responses now look like this:

"cc": [
    {
        "email": "daniel.l.gerber@gmail.com",
        "name": "Daniel Gerber"
    },
    {
        "email": "narenst@gmail.com",
        "name": "Naren"
    },
    {
        "email": "townley.andrew@gmail.com",
        "name": "Andrew Townley"
    }
],

Instead of this:
"cc": [
    [
        "Daniel Gerber",
        "daniel.l.gerber@gmail.com"
    ],
    [
        "Naren",
        "narenst@gmail.com"
    ],
    [
        "Andrew Townley",
        "townley.andrew@gmail.com"
    ]
],

Test Plan: Manual testing

Reviewers: mg

Reviewed By: mg

CC: bengotow

Differential Revision: https://review.inboxapp.com/D132