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/forms.py

@@ -12,10 +12,10 @@ from django.utils.translation import ugettext_lazy as _
 from common.forms import DynamicModelForm
 
 from .classes import WorkflowAction
+from .fields import WorfklowImageField
 from .models import (
     Workflow, WorkflowState, WorkflowStateAction, WorkflowTransition
 )
-from .widgets import WorkflowImageWidget
 
 
 class WorkflowActionSelectionForm(forms.Form):
@@ -178,9 +178,9 @@ class WorkflowInstanceTransitionForm(forms.Form):
 
 
 class WorkflowPreviewForm(forms.Form):
-    preview = forms.IntegerField(widget=WorkflowImageWidget())
+    workflow = WorfklowImageField()
 
     def __init__(self, *args, **kwargs):
         instance = kwargs.pop('instance', None)
         super(WorkflowPreviewForm, self).__init__(*args, **kwargs)
-        self.fields['preview'].initial = instance
+        self.fields['workflow'].initial = instance