Add test for backdrop not being called on update
Do not call backdrop on data-set updates

[fixes #67930714]
https://www.pivotaltracker.com/story/show/67930714

Backdrop create data-set should only be called when creating a new
data-set. It should not be called when udpating an existing data-set.
Merge pull request #87 from alphagov/prevent-backdrop-on-update

Do not call backdrop on data-set updates
Also run doctests in run_tests.sh
Merge pull request #86 from alphagov/also-run-doctests

Also run doctests in run_tests.sh
Add tests for long cache headers on api views

[#58614074]
https://www.pivotaltracker.com/story/show/58614074
Add long cache headers for api views

[delivers #58614074]
https://www.pivotaltracker.com/story/show/58614074

Set very long cache headers (365 days) on api views. This is so that
they can be served from varnish even if the backend app goes down.
Additionally a Vary header has been set on the Authorization header.
This is so that we don't risk sending authenticated results to
unauthenticated clients.
Merge pull request #83 from alphagov/long-cache-api-view

Add long cache headers for api views
Only clean up py files inside stagecraft/

Before we run tests, we clean up python bytecode files. We should only
do this inside the django project, otherwise we traverse the venv/ and
other directories which is a waste of time.
Merge pull request #89 from alphagov/speed-up-run-tests

Only clean up py files inside stagecraft/
Add (failing) tests for /_status/data-sets
Add healthcheck URL to check data sets

The purpose of this endpoint is to:

- check we can meaningfully query the database for data sets
- provide a basic description of what we found (sanity check)

See https://www.pivotaltracker.com/story/show/58614078 [#58614078]
Merge pull request #90 from alphagov/data-sets-health-check

Add healthcheck URL to check data sets
Minor PEP8 warnings fixes etc.
Change the way one of our tests works

We have a test which emulates a problem in the `save()` method of a
DataSet. This commit changes the way it works...
Tests & implementation for purge_varnish library
Tests & implementation for calculate_purge_urls

From a given DataSet, calculate all the possible URLs that could refer
to that DataSet. This allows us to purge the right URLs.
(Failing) test that the DataSet purges on update
Call purge when DataSet is updated.

Use the new `calculate_purge_urls` helper code and `purge_varnish`
library to send a Varnish purge request when a DataSet is `save()`d
Merge pull request #88 from alphagov/data-set-purging

Purge Varnish cache when DataSets are updated