Add support for enabling subscriptions in GraphiQL on Django through a flag on the BaseView class

[lijok]

Description

Following on from this change #344 (thanks @jkimbo for the reference), I would like to propose allowing subscriptions to be enabled in GraphiQL for Django through a flag.
Without this, as a user I have to define a new AsyncGraphQLView as seen in the example here https://github.com/strawberry-graphql/examples/blob/main/django-subscriptions/api/views.py, to enable subscriptions, where as I would otherwise not need to do so.

Types of Changes

• Core
• Bugfix
• New feature
• Enhancement/optimization
• Documentation

Issues Fixed or Closed by This PR

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• I have tested the changes and verified that they work and don't break anything (as well as I can manage).

[botberry]

Thanks for adding the RELEASE.md file!

Here's a preview of the changelog:

---

This release adds support for enabling subscriptions in GraphiQL
on Django by setting a flag subscriptions_enabled on the BaseView class.

from strawberry.django.views import AsyncGraphQLView

from .schema import schema

urlpatterns = [path("graphql", AsyncGraphQLView.as_view(schema=schema, graphiql=True, subscriptions_enabled=True))]

---

Here's the preview release card for twitter:

Here's the tweet text:

🆕 Release (next) is out! Thanks to lijok for the PR 👏

Get it here 👉 https://github.com/strawberry-graphql/strawberry/releases/tag/(next)

[codecov]

Codecov Report

Merging #1215 (a41ebb2) into main (503d906) will increase coverage by 0.00%.
The diff coverage is 100.00%.

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  Coverage   97.55%   97.56%           
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  Files          88       88           
  Lines        3319     3321    +2     
  Branches      477      477           
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+ Hits         3238     3240    +2     
  Misses         45       45           
  Partials       36       36           

[jkimbo]

Lgtm