Handle bulk deletion of Workflows, Analyses, and Datasets properly

[scottx611x]

Commit: fbc5a7b

Steps to reproduce

• Perform a QuerySet delete() upon one of the models mentioned.

Observed behavior

• If you debug the deletion process you can see that to achieve the proper deletion checking, the QuerySet is iterated over and each object is individual checked if it can be deleted. Then each object that passes these checks is deleted using the Model's delete() method.

Expected behavior

• The checks that are in place to see if Workflows, Analyses, and Datasets can be deleted should remain, but there should be some logic introduced to perform a bulk deletion on the objects that pass the checks as individual deletions could become costly in the long run.

[hackdna]

see also: #1425, #1428 and #480

[hackdna]

Simply use pre-delete or post-delete signal handlers.
https://docs.djangoproject.com/en/1.8/topics/db/models/#overriding-predefined-model-methods