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):

app/models/service.py

@@ -5,7 +5,7 @@ from flask import abort, current_app
 from notifications_utils.timezones import local_timezone
 from werkzeug.utils import cached_property
 
-from app.models import EmptyModelList, JSONModel
+from app.models import JSONModel
 from app.models.job import (
     ImmediateJobs,
     PaginatedJobs,
@@ -123,13 +123,13 @@ class Service(JSONModel):
     @cached_property
     def immediate_jobs(self):
         if not self.has_jobs:
-            return EmptyModelList()
+            return []
         return ImmediateJobs(self.id)
 
     @cached_property
     def scheduled_jobs(self):
         if not self.has_jobs:
-            return EmptyModelList()
+            return []
         return ScheduledJobs(self.id)
 
     @cached_property