Replace all instances of unicode only handling to use force_text.

Replace all __unicode__ methods to __str__ and the
@python_2_unicode_compatible decorator.
Replace all instance of smart_str, smart_unicode, force_uncode
with force_text.

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

mayan/apps/document_indexing/widgets.py

@@ -2,6 +2,7 @@
 from __future__ import unicode_literals
 
 from django.apps import apps
+from django.utils.encoding import force_text
 from django.utils.html import mark_safe, escape
 
 
@@ -50,7 +51,7 @@ def node_level(node):
             [
                 '     ' * node.get_level(),
                 '' if node.is_root_node() else '<i class="fa fa-level-up fa-rotate-90"></i> ',
-                unicode(node)
+                force_text(node)
             ]
         )
     )