Add widget support to SourceColumn

Allow passing a widget class to SourceColumn. This makes
using lambdas to render model column unnecesary and are
mostly removed too.

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

mayan/apps/documents/migrations/0050_duplicateddocumentproxy.py

@@ -0,0 +1,27 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.16 on 2018-12-22 09:30
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('documents', '0049_auto_20181211_0011'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='DuplicatedDocumentProxy',
+            fields=[
+            ],
+            options={
+                'verbose_name': 'Duplicated document',
+                'proxy': True,
+                'verbose_name_plural': 'Duplicated documents',
+                'indexes': [],
+            },
+            bases=('documents.document',),
+        ),
+    ]