Refactor the workflow preview generation to work as a
background task API service. Solves GitLab issue #532.
The image generation runs as an out of process task
ensuring that the HTTP request is never compromised.
A new task queue named "document_states_fast" was created.
The settings WORKFLOWS_IMAGE_CACHE_TIME,
WORKFLOWS_IMAGE_CACHE_STORAGE_BACKEND,
WORKFLOWS_IMAGE_CACHE_STORAGE_BACKEND_ARGUMENTS we added.
Images generated are stored by default under /mayan/media/workflows.
The Dockerfile and deployment instructions are updated
to include the new queue.
Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>
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