Hastexo-xblock

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* [Enhancement] Create admin page for stacks
* [Bug fix] Avoid unhandled exception on SSH close

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* [Enhancement] Relax dependency constraints

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* [Bug fix] Fix JSONField model and migration
* [Bug fix] Handle null `hook_events` properly

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* [Enhancement] tox: Bump xblock12 env to XBlock 1.2.3
* [Enhancement] Add Python 3.7 to Travis configuration
* [Enhancement] tasks.py: continue on specific EnvironmentErrors during SSH connection
* [Enhancement] Update devstack documentation
* [Enhancement] Task hooks
* [DEPRECATION] `reboot_on_resume` will be removed in a future release, as its
intended purpose is now better served by task hooks.
* [CONFIGURATION] The `suspend_in_parallel` configuration option is now a NOOP,
as suspension now always happens in parallel via simultaneously running
Celery tasks.

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* [Bug fix] Fix XML parsing backward compatibility

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* [BACKWARD INCOMPATIBLE FOR GCP LABS ONLY] Encode GCP stack names
* [Enhancement] Provider stack listing
* [Enhancement] Reaper zombie destroyer
* [Enhancement] Add Python 3.7 test target