1980: Fixing issue

* Only allow post publish edits if the last
version exists, is public and there's at least
a task for the next step done.
* Any subtitling work is should be defined by 
the number of subtitles typed on the editor
not if they are the same as the initial ones (this
can mess with youtube subtitle imports)
Fix descriptions when we save subtitle to the local storage.
Merge branch 'dev' of github.com:pculture/unisubs into dev