Proof of concept of the workflow instance context

Add support for workflow instance JSON context.
Add support for two step workflow transition.
Add support for dynamic form creation for transition execution.

Signed-off-by: Roberto Rosario <roberto.rosario@mayan-edms.com>

mayan/apps/document_states/migrations/0014_auto_20190630_1331.py

@@ -0,0 +1,40 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.20 on 2019-06-30 13:31
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('document_states', '0013_auto_20190423_0810'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='WorkflowTransitionField',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('name', models.CharField(max_length=128, verbose_name='Internal name')),
+                ('label', models.CharField(max_length=128, verbose_name='Label')),
+                ('help_text', models.TextField(blank=True, verbose_name='Help text')),
+                ('required', models.BooleanField(default=False, verbose_name='Required')),
+                ('transition', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='fields', to='document_states.WorkflowTransition', verbose_name='Transition')),
+            ],
+            options={
+                'verbose_name': 'Workflow transition trigger event',
+                'verbose_name_plural': 'Workflow transitions trigger events',
+            },
+        ),
+        migrations.AddField(
+            model_name='workflowinstance',
+            name='context',
+            field=models.TextField(blank=True, verbose_name='Backend data'),
+        ),
+        migrations.AlterUniqueTogether(
+            name='workflowtransitionfield',
+            unique_together=set([('transition', 'name')]),
+        ),
+    ]