Rename 'all' metascope to more descriptive 'inherit'

since it means 'inheriting' the owner's permissions

'all' prompted the question 'all of what, exactly?'

Additionally, fix some NameErrors that should have been KeyErrors

docs/source/rbac/roles.md

@@ -123,13 +123,13 @@ has,
 define the `server` role.
 
 To restore the JupyterHub 1.x behavior of servers being able to do anything their owners can do,
-use the scope `all`:
+use the scope `inherit` (for 'inheriting' the owner's permissions):
 
 ```python
 c.JupyterHub.load_roles = [
  {
    'name': 'server',
-   'scopes': ['all'],
+   'scopes': ['inherit'],
  }
 ]
 ```