Better check for upload_filters

- Stagecraft may pass this value through as `""` or `[]` or `None` and `if not upload_filters` works for all these cases.
Merge pull request #269 from alphagov/always_add_a_default_upload_filter

Better check for upload_filters
WIP - adding feature test for delete end point
Add in api_common for shared helper functions
Add in delete feature test
Add in api_common
Ensure a data-set exists before deleting it

- This is a slightly stupid hack, in which I post to a data-set to create it, then delete it so the delete flow works
- LOL?
Change 400 to a 404

- If you try to delete a data-set and it doesn't exist, return a 404
- 400 Bad Request seemed wrong, because it's a perfectly well formed request, just trying to delete something that doesn't exist.
- This is a really silly semantics thing, and I couldn't find any reliable information on it really, but I did go to the wwws http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html
Add a new message parsing step

- The old message parsing step seemed to disregard the message, which seemed nuts.
- Then I thought, actually, I'm just an idiot, I'm missing something.
- But then I couldn't figure out what that was.
- So I wrote a new method that really explicitly just looks for the message.
- And I didn't delete the old one :-/
[#68811824] Feature tests for deleting things

- Test that we get a good code and message back if you try and delete something that doesn't exist
- Test that deleting things gives a 200 and a message back
- Tagged with `@delete_things` so you can run behave tests on this tiny subcategory really fast (woo) ala:
```
behave --tags=@delete_things
```
Merge branch 'master' into delete_datasets

* master:
  Better check for upload_filters
[#68811824] Update token constant

- Works with pp-deployment, https://github.gds/gds/pp-deployment/pull/231
- `CREATE_COLLECTION_ENDPOINT_TOKEN` is now `STAGECRAFT_COLLECTION_ENDPOINT_TOKEN`
- `_allow_delete_collection` and `_allow_create_collection` are now convenience methods that reference `_allow_modify_collection`, seeing as stagecraft shuold be able to do both
Merge branch 'delete_datasets' of github.com:alphagov/backdrop into delete_datasets

* 'delete_datasets' of github.com:alphagov/backdrop:
  [#68811824] Feature tests for deleting things
  Add a new message parsing step
  Change 400 to a 404
  Ensure a data-set exists before deleting it
  Add in api_common
  Add in delete feature test
  Add in api_common for shared helper functions
  Add feature test for delete end point