Django-comments-xtd

* Fixes issue when the 'sent' view does not receive a 'c' query string parameter. See [PR-305](https://github.com/danirus/django-comments-xtd/pull/305). Thanks to dest81.

* Drops support for Django 2.0 and 2.1.
* Requires django-contrib-comments >= 2.1, and djangorestframework >= 3.12.
* Fixes warning when generating the OpenAPI schema. Thanks to ivanychev. Seee[PR-296](https://github.com/danirus/django-comments-xtd/pull/296).
* Fixes issue with `render_xtdcomment_tree` templatetag, thanks to dest81. See [PR-295](https://github.com/danirus/django-comments-xtd/pull/295).
* Fixes issue 291, about the frontend plugin not being aware of the setting COMMENTS_XTD_DEFAULT_FOLLOWUP. It also fixes the content of the `login_url` props attribute. Its value is now the content of `settings.LOGIN_URL`.
* Fixes issue 284, about sending a comment twice by clicking the comment send button twice. It happened when not using the JavaScript plugin.

* Fixes issue 292 with the workflow upload-pypi.yml, that caused the package bundle to be built without JavaScript files.

* Adds Italian translation (thanks to dlrsp-dev).
* Fixes issue 279, about a syntax mistake in the get_flags function, in the models.py module. Thanks to manisar2.
* Fixes issue 271, about an issue with django-comments-xtd data migration 0008. The fix consists of making the migration non-effective. It also requires that the django project developer runs the management command `initialize_nested_count` manually, so that the field `nested_count` of `XtdComment` gets populated with the correct value. The command is idempotent. Thanks to jxcl.

* Adds new setting COMMENTS_XTD_DEFAULT_FOLLOWUP, which is used to
initialize the follow-up form field. By default its value is False. Thanks to drholera. Closes ticket 206.
* Fixes issue 274, about wrong validation of fields name and email in the
WriteCommentSerializer. Thanks to dest81.

* Fixes issue 248, about the API returning comments' submit_date in UTC
when the setting USE_TZ is enabled and a different TIME_ZONE is given.
Thanks to Loneattic.
* Fixes issue 250, which reports that using the web API to post a comment
with a reply_to field that would break the max_thread_level should not
produce an exception but rather a controlled response with an appropriate
HTTP code. Thanks to impythonista.
* Fixes issue 255, about the web API not returning the comment ID when
creating a new comment. Thanks to mhoonjeon.
* Fixes issue 256, about an issue in the JavaScript plugin that displays
the "reply" link even when the max_thread_level has been reached.
Thanks to odescopi.