Hastexo-xblock

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* [Enhancement] Allow overriding settings for the
`hastexo_guacamole_client` from a configuration file by defining
it as `HASTEXO_GUACAMOLE_CFG`.

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* [BREAKING CHANGE] Replace Guacamole servlet with a Django ASGI
application, which uses Django-Channels and the
[pyguacamole](https://pypi.org/project/pyguacamole/) library.
As of 5.0, deployment of this XBlock will no longer rely on the
Apache Tomcat servlet container, which Guacamole normally uses, but
instead on the Daphne ASGI server. If you have been deploying this
XBlock with the modified edx-configuration playbooks as explained in
the README, deployment should still be automatic for you. It is,
however, strongly advised that you respin your app servers from
scratch, in order to keep any residual Tomcat servlet configuration
from lingering on them.

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* [Bug fix] When deleting learner state, the `stack_name` value
gets wiped out. If we then try to update the stack (for example
via `keepalive`) we get `Stack.DoesNotExist` error. Check if
`stack_name` has a value before attempting to update and if not,
set it again.

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* [Bug fix] Implement more of `ScorableXBlockMixin` functionality
for using the grading related instructor tasks like overriding
and rescoring learner's submissions.

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* [Bug fix] Make the XBlock a subclass of `ScorableXBlockMixin` so
grades would be calculated correctly for each subsection.
* [Enhancement] Make progress check wait dialog wording more general
and more suitable for different progress check configurations.

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[Bug fix] Fix export error, when provider attributes are not defined.