test(rxModalForm): Use `submitText` and `cancelText` in our demo page

We weren't testing that `submitText` or `cancelText` work on the
`rxModalForm`. This changes the demo page to use both of them, and
adds midway tests to confirm.
Merge pull request #551 from rackerlabs/rxModal-submit-text-test

test(rxModalForm): Use `submitText` in our demo page