Use SortByName mixin where possible

Implementing `__lt__` makes objects sortable.

Rather than reimplementing `__lt__` each time we want to make an object
sortable by name, we can inherit from the extant `SortByNameMixin`.

app/broadcast_areas/models.py

@@ -8,21 +8,17 @@ from rtreelib import Rect
 from werkzeug.utils import cached_property
 
 from app.formatters import square_metres_to_square_miles
+from app.models import SortByNameMixin
 
 from .populations import CITY_OF_LONDON
 from .repo import BroadcastAreasRepository, rtree_index
 
 
-class SortableMixin:
+class SortableMixin(SortByNameMixin):
 
     def __repr__(self):
         return f'{self.__class__.__name__}(<{self.id}>)'
 
-    def __lt__(self, other):
-        # Implementing __lt__ means any classes inheriting from this
-        # method are sortable
-        return self.name < other.name
-
     def __eq__(self, other):
         return self.id == other.id
 

app/models/organisation.py

@@ -64,9 +64,6 @@ class Organisation(JSONModel, SortByNameMixin):
             return cls({})
         return cls(organisations_client.get_organisation(org_id))
 
-    def __lt__(self, other):
-        return self.name.lower() < other.name.lower()
-
     @classmethod
     def from_domain(cls, domain):
         return cls(organisations_client.get_organisation_by_domain(domain))