Changed image widget name from 'image-widget' to 'image'

Summary: Naming the image widget 'image-widget' is a little inconsistent with how we name our
other widgets; it'd be better if it were simply 'image'.

Test Plan:
- Run webapp.
- Verify that the image widget operates as expected: change the ranges, add labels, permalink + reload, etc.
- Here's a [sample image](http://localhost:9000/test.html#content=%7B%22question%22%3A%7B%22content%22%3A%22%5B%5B%E2%98%83%20image%201%5D%5D%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%22image%201%22%3A%7B%22type%22%3A%22image%22%2C%22graded%22%3Atrue%2C%22options%22%3A%7B%22backgroundImage%22%3A%7B%22url%22%3A%22https%3A%2F%2Fka-perseus-graphie.s3.amazonaws.com%2Fb9f5b3b8b6ed17e61f6612e7c0af679ac0853bf0.png%22%2C%22width%22%3A400%2C%22height%22%3A400%7D%2C%22box%22%3A%5B400%2C400%5D%2C%22labels%22%3A%5B%7B%22content%22%3A%22Hello%22%2C%22coordinates%22%3A%5B3%2C0%5D%2C%22alignment%22%3A%22center%22%7D%2C%7B%22content%22%3A%22Goodbye%22%2C%22coordinates%22%3A%5B4%2C-2%5D%2C%22alignment%22%3A%22right%22%7D%5D%2C%22range%22%3A%5B%5B0%2C5%5D%2C%5B-5%2C3%5D%5D%7D%7D%7D%7D%2C%22answerArea%22%3A%7B%22type%22%3A%22multiple%22%2C%22options%22%3A%7B%22content%22%3A%22%22%2C%22images%22%3A%7B%7D%2C%22widgets%22%3A%7B%7D%7D%2C%22calculator%22%3Afalse%7D%2C%22hints%22%3A%5B%5D%7D).

Auditors: jack