handle ZeroDivisionError exceptions, occurs when the height  and thumbnail height are the same
for large size submissions, ensure attachments are saved, as the submission is treated as a duplicate
get full filepath of an attachment
create zip file from Attachments model
refactored attachment zip to a utils function
add zip export type and mime
generate attachments zip export function
task to generate attachments zip export
delete zip exports after a specified period
link to zip export list
separated kml export data generation into a function of its own
async kml export support
Merge branch 'master' into 952-large-submissions

* master:
  Implemented language changes on column picker - fixes #955
  Updated query mongo to enclose query params in an  - fixes #622
  typo in comment
  Dont persist filtered exports to db
  Used old test runner just to be sure
  Temporarily set test test runner to default django runner to make sure nothing is broken by export changes
  Updates csv and xls export to share a data export view and tests to use anmed url for csv_export and xls_export
  Re-use existing export when data has not been updated
  Also check deleted surveys when getting the last update time
  Return 404 if no records match query criteria
  Added function to submit with editable transport instances
  Added time of last submission getter
  Dont explicitly convert to unicode
  Fixture for unicode characters in relevant column test
  Pass error as as and dont try to convert to unicode, it is propely rendered
  Added function to check if the message is unicode before trying to convert
  Explictly use message attribute of validation errors and encode to utf-8 incase message contains non-ascii characters

Conflicts:
	odk_viewer/views.py
	settings.py
return a valid OpenRosaResponse for the head request
pep8 cleanup, raise exceptions
pep8 cleanup
Merge pull request #1027 from modilabs/952-large-submissions

Handle large submissions and async zip exports - closes #952, closes #846.
Merge with master