Issue/editor 344 media upload blocker

Fixes https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/344
as well as part of https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/307

This is a stop gap solution that will be "upgraded" with:

https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/307

The basic point here is to not allow the user to modify/leave the post editing screen until the media has completed uploading or the the user explicitly cancelled the upload by tapping on the "Uploading media" spinner.

In addition there is a fix in this PR that removes the local URL issue we have been seeing on saved posts.

/cc @diegoreymendez