Move add_to_class functions to their own module

* The new module is called methods.py and found on each app.
* Add keyword arguments to add_to_class instances.
* Remove catch all exception handling for the check in and
  check out views.
* Improve checkouts tests code reducing redundant code.

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

mayan/apps/cabinets/methods.py

@@ -0,0 +1,20 @@
+from __future__ import unicode_literals
+
+from django.apps import apps
+from django.utils.translation import ugettext_lazy as _
+
+
+def method_get_document_cabinets(self):
+    DocumentCabinet = apps.get_model(
+        app_label='cabinets', model_name='DocumentCabinet'
+    )
+
+    return DocumentCabinet.objects.filter(documents=self).order_by(
+        'parent__label', 'label'
+    )
+
+
+method_get_document_cabinets.help_text = _(
+    'Return a list of cabinets containing the document'
+)
+method_get_document_cabinets.short_description = _('get_cabinets()')