Make CSRF cookie and header names configuration based on django settings

[BobReid]

Issue:

Django allows the the CSRF cookie and header to be configured through the use of settings variables. graphene-django's implementation if GraphIQL does not respect these settings which causes a CSRF failure.

Fix:
This PR fixes the issue by reading the values from the django settings values and uses them in the graphiql template.

Repro:

1. create graphene-django project
2. set django setting CSRF_COOKIE_NAME and CSRF_HEADER_NAME to non standard values
3. Launch graphiql

Expected:
GraphIQL works as expected (request / response pass and scheme can be inspected)

Actual:
403 status' are returned due to a CSRF failure

[coveralls]

Coverage increased (+0.03%) to 94.547% when pulling c8db1fe on GetResQ:make_csrf_token_configurable into f76f38e on graphql-python:master.

[dulmandakh]

@BobReid hey, could you please rebase or resolve the conflicts. I would like to test and review your changes. Thank you

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[BobReid]

@dulmandakh

I was going to rebase these changes but it looks like the GraphIQL javascript has been extracted from the html template and placed into a static file. This refactor is incompatible with my approach because I was relying on running the javascript through the templating engine in order to set the header name dynamically.

I could stuff the header name in a global JS variable in the html template and read it in the GraphIQL script. It is ugly but would work.

Thoughts?

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.