Disable new media picker in favor of something simpler

The new media picker isn't ready for prime time. This pull request effectively disables it in favor of a simpler approach:

https://cloudup.com/i7ZtVhwb0eC

A new library is used for multiple photo selection (`CTAssetsPickerController`). Some things to note:
* All code in the new `WPMediaProcessor` was factored out of the new media library and repurposed to be more general. In other words, I didn't write the actual image processing logic.
* Uploading videos has been disabled for now (and corresponding video settings removed) until we have a video upload experience that works.
* The "Always Ask" setting for image sizes has been removed for now, but otherwise the image size setting is still respected.
* There's still no good indication that a photo is being uploaded. This sucks, and we should find a way to address it when we swap in a visual editor.
* In general, error handling for photo uploads is still not very good.