Hastexo-xblock

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* [Enhancement] Be more specific when raising the exception for
restricting lab access due to lab usage limit being reached.
* [Enhancement] Add an option to track and limit a learners lab
usage. To support time tracking, link a learner to their stacks
across the platform by adding a Foreign Key field for user to
the Stack object.
Add configuration options for setting a time limit for using labs
(`lab_usage_limit`) in seconds and how to handle a breach of the
set limit (`lab_usage_limit_breach_policy`).
* [Testing] Include XBlock 1.6 in the test matrix.

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* [Documentation] Update README with improved instructions for Open
edX Maple (using Tutor) and Lilac (using edx-configuration).

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* [Bug fix] Don't fail to run if a listed provider is not
configured. Allow to move on to the next provider and log a
warning message for the provider initialisation failure.
* [BREAKING CHANGE] Update the `GUACD_*` environment variables to
better suit a Tutor deployment. Rename the variables to
`GUACD_SERVICE_HOST` and `GUACD_SERVICE_PORT` to directly read
values set for the guacd service with the `tutor k8s` deployment.
Update the default values to support the `tutor local`/
`tutor dev` deployment.
* [BREAKING CHANGE] Update dependencies that should be in sync with
`edx-platform` to support the `Maple` release.
As of 6.0, this XBlock only supports Open edX versions Maple and
higher. As the community has switched the supported deployment method
from edx-configuration playbooks to Tutor, this XBlock can also
be deployed with Tutor only. Instructions for the latter can be found
in the README.
* [Enhancement] Add support for Tutor deployment, by dropping the
`wait_for_ping` logic.
* [Enhancement] Add support for Apache Guacamole version `1.4.0`,
make it the new default.
* [Enhancement] Make the `guacamole-common-js` library version
configurable by the `guacamole_js_version` setting.
* [Bug fix] Fix unbalanced tags (`<p>` vs. `<div>`) in the static
`main.html` template.
* [Testing] Add basic HTML validation for static templates.

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* [Enhancement] Add constraints to Django version requirement for
the `hastexo_guacamole_client`.

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* [Bug fix] Make sure that any error message that is added to the
`error_msg` field of a stack, gets truncated before a stack update.
* [Testing] Include XBlock 1.5 in the test matrix, remove XBlock 1.3.
* [Testing] Include Python 3.9 in the test matrix, remove remnants of
Python 3.5 test coverage.

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* [Bug fix] Truncate the error message for `LaunchError` to fit
256 characters and thus, could be added to the `error_msg` field
of a `Stack`.