add deploy:cleanup_lingering_releases task

Adding a new task for cleaning up lingering releases. Detailed comments as to its purpose in the code.
There should never be more than one, as update_code is how you add a new release (I am assuming its the only way, otherwise you need to hook in this task to any other tasks that do it). In order for this to be the case you'd need to run update_code and add a new release, without it deleting any existing lingering ones.