Temporarily disable REST for Jetpack sites

It just doesn't work when the JSON API module is disabled.
Also, there are some issues where failed uploads don't show any error.

Failed upload before:

![img_0609](https://cloud.githubusercontent.com/assets/8739/4012519/8ad2c3d4-2a0e-11e4-8b25-f949750cdda6.PNG)

Failed upload after:

![img_0608](https://cloud.githubusercontent.com/assets/8739/4012523/93579624-2a0e-11e4-974a-8e451d73d205.PNG)

refs #2258