Add a new custom icon set to the icons app

apps/icons/sets/custom.py

@@ -0,0 +1,12 @@
+from django.utils.translation import ugettext_lazy as _
+
+from icons.literals import *
+
+PATH = 'custom'
+ID = 'custom'
+LABEL = _(u'Custom')
+
+DICTIONARY = {
+    FILE_EXTENSION_ERROR: 'file_extension_error.png',
+    FILE_EXTENSION_UNKNOWN: 'file_extension_unknown.png'
+}