enable djcelery task info views
change entity locking implementation

Setting a 'locked' bit in entity.json has been problematic.
Instead write celery task_id to a file on lock, and remove the
file on unlock.  Write the task_id to the file so other processes
can see what task is pending.
refactor entity-add-file on abstract Task class; change locking

Rework entity_add_file task to use Celery abstract class,
to take advantage of after_return and on_(success|failure)
handlers.
Lock entity right after calling in view (once have a task_id)
and unlock it in after_return handler.
Hopefully this will prevent errors within entity-add-file
from leaving the entity in a locked state.

The new locking should make it possible for pages viewed
while a task is pending to retrieve the task_id so they can
see what's going on.

See "Abstract classes" section of
http://celery.readthedocs.org/en/latest/userguide/tasks.html