Workflows: Refactor workflow preview generation

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>

mayan/apps/document_states/fields.py

@@ -0,0 +1,9 @@
+from __future__ import absolute_import, unicode_literals
+
+from django import forms
+
+from .widgets import WorkflowImageWidget
+
+
+class WorfklowImageField(forms.fields.Field):
+    widget = WorkflowImageWidget