Use django-djconfig for kfet app.

Old configuration(/settings), based on Settings model, system is
deleted: SettingsForm, Settings.
New system use `django-djconfig` module.

- `kfet.config` module provides `kfet_config` to access configuration concerning
kfet app.
- Views, forms, models, etc now use this object to retrieve conf values.
- Views no longer add config values to context, instead templates use
  `kfet_config` provided by a new context_processor.
- Enhance list and update views of settings.
- Fix: settings can directly be used without having to visit a specific
  page...

Misc
- Delete some py2/3 imports
- Delete unused imports in kfet.models and kfet.views
- Some PEP8 compliance

kfet/config.py

@@ -0,0 +1,25 @@
+# -*- coding: utf-8 -*-
+
+from djconfig import config
+
+
+class KFetConfig(object):
+    """kfet app configuration.
+
+    Enhance dependency with backend used to store config.
+    Usable after DjConfig middleware was called.
+
+    """
+    prefix = 'kfet_'
+
+    def __getattr__(self, key):
+        dj_key = '{}{}'.format(self.prefix, key)
+        return getattr(config, dj_key)
+
+    def list(self):
+        from kfet.forms import KFetConfigForm
+        return [(field.label, getattr(config, name), )
+                for name, field in KFetConfigForm.base_fields.items()]
+
+
+kfet_config = KFetConfig()