Refactor the converter app

Don't cache the entire converter class to lower memory usage.
Instead a get_converter_class() function is now provided to
load the converter backend class.

Add model permission inheritance to transformations to
removel custom permission checking code in the views.

User keyword arguments.

Update URL parameters to the '_id' form.

Add missing edit and delete icons. Improve the create
icon using composition.

Update add to comply with MERCs 5 and 6.

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

mayan/apps/converter/utils.py

@@ -0,0 +1,13 @@
+from __future__ import unicode_literals
+
+import logging
+
+from django.utils.module_loading import import_string
+
+from .settings import setting_graphics_backend
+
+logger = logging.getLogger(__name__)
+
+
+def get_converter_class():
+    return import_string(dotted_path=setting_graphics_backend.value)