Bug 1097673 - Implement Create room screen

Should we call `LoadingOverlay.hide()` in every `resolve` and `reject`?  Now it's called just once in [1]

[1] https://github.com/mozilla-b2g/firefoxos-loop-client/pull/314/files#diff-08c24c25d6529b8d2da6fc1e26ff2c96R90