Implement __radd__ on ModelList to make addition commutative

Because ModelList implements `__add__` we can do the following:

```python
ImmediateJobs() + ScheduledJobs()
ImmediateJobs() + []
```

Both of these call the `__add__` method of `ImmediateJobs`.

What we can’t do is this:
```python
[] + ScheduledJobs()
```

That tries to call the `__add__` method of list, which doesn’t know what
to do with an instance of `ModelList`.

The Pythonic way to deal with this is to implement `__radd__` (right
add) which is invoked when our instance is on the right hand side of the
addition operator.

app/models/__init__.py

@@ -75,13 +75,8 @@ class ModelList(ABC, Sequence):
     def __add__(self, other):
         return list(self) + list(other)
 
-
-class EmptyModelList(ModelList):
-
-    client_method = model = None
-
-    def __init__(self, *args):
-        self.items = []
+    def __radd__(self, other):
+        return list(other) + list(self)
 
 
 class InviteTokenError(Exception):