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/common/utils.py

@@ -11,6 +11,7 @@ from django.conf import settings
 from django.core.urlresolvers import resolve as django_resolve
 from django.urls.base import get_script_prefix
 from django.utils.datastructures import MultiValueDict
+from django.utils.encoding import force_text
 from django.utils.http import urlquote as django_urlquote
 from django.utils.http import urlencode as django_urlencode
 
@@ -136,7 +137,7 @@ def return_attrib(obj, attrib, arguments=None):
         if settings.DEBUG:
             return 'Attribute error: %s; %s' % (attrib, exception)
         else:
-            return unicode(exception)
+            return force_text(exception)
 
 
 def urlquote(link=None, get=None):