Make existing_keys a required argument

Because we should always expect its presence, even it’s an empty list.
And because having mutable default arguments can have nasty side
effects.

app/main/forms.py

@@ -525,7 +525,7 @@ class ChooseTimeForm(StripWhitespaceForm):
 
 
 class CreateKeyForm(StripWhitespaceForm):
-    def __init__(self, existing_keys=[], *args, **kwargs):
+    def __init__(self, existing_keys, *args, **kwargs):
         self.existing_key_names = [
             key['name'].lower() for key in existing_keys
         ]