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/user_management/utils.py

@@ -0,0 +1,18 @@
+from __future__ import unicode_literals
+
+from django.apps import apps
+from django.contrib.auth import get_user_model
+
+
+def get_groups():
+    Group = apps.get_model(app_label='auth', model_name='Group')
+    return ','.join([group.name for group in Group.objects.all()])
+
+
+def get_users():
+    return ','.join(
+        [
+            user.get_full_name() or user.username
+            for user in get_user_model().objects.all()
+        ]
+    )