Add GraphiQL Explorer plugin

[kiendang]

Close #1204.

Based on graphiql-plugin-explorer CDN example and strawberry graphiql template.

[firaskafri]

@kiendang thank you! Would you please check the tests?

[kiendang]

@firaskafri looks like someone, not sure if it's you, triggered a test re-run and they all pass now.
tests all pass for me locally too. doesn't seem like the changes are related to the failure either.

[sjdemartini]

I tested this locally with the example app in examples/cookbook-plain, and it looks great to me, thanks for adding this @kiendang!

I changed the examples/cookbook-plain/requirements.txt to the following to use the current local graphene-django code instead of the published version:

graphene
django~=3.2
../../  # graphene-django