Block HTML view during uploads

Fixes https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/351

We now block the user from seeing the HTML view until the entire upload process is complete. This is a stop gap solution for 4.6 and will probably be addressed again in https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/376

/cc @diegoreymendez 
This line brings me happiness.  So much more readable! :+1: