Refactor permissions handling to use sets

We didn’t like the nested conditional way of doing this. So this commit
refactors the way that permissions are set by:

- splitting it up into multiple, clearly named methods
- treating the list of permissions as `set`s, which they naturally are,
  because you can’t have duplicate permissions (this removes a lot of
  the complexity around having to test for membership before removing
  a permission, for example)

app/main/views/service_settings.py

@@ -224,26 +224,36 @@ def service_switch_research_mode(service_id):
     return redirect(url_for('.service_settings', service_id=service_id))
 
 
-def switch_service_permissions(service_id, permission, sms_sender=None, force=None):
+def switch_service_permissions(service_id, permission, sms_sender=None):
 
-    permissions = current_service['permissions'].copy()
+    force_service_permission(
+        service_id,
+        permission,
+        on=permission not in current_service['permissions'],
+        sms_sender=sms_sender
+    )
 
-    if force is None:
-        if permission in permissions:
-            permissions.remove(permission)
-        else:
-            permissions.append(permission)
-    else:
-        if force is True:
-            permissions.append(permission)
-        elif force is False and permission in permissions:
-            permissions.remove(permission)
 
-    current_service['permissions'] = list(set(permissions))
+def force_service_permission(service_id, permission, on=False, sms_sender=None):
+
+    permissions, permission = set(current_service['permissions']), {permission}
+
+    update_service_permissions(
+        service_id,
+        permissions | permission if on else permissions - permission,
+        sms_sender=sms_sender
+    )
+
+
+def update_service_permissions(service_id, permissions, sms_sender=None):
+
+    current_service['permissions'] = list(permissions)
+
+    data = {'permissions': current_service['permissions']}
 
-    data = {'permissions': permissions}
     if sms_sender:
         data['sms_sender'] = sms_sender
+
     service_api_client.update_service_with_properties(service_id, data)
 
 
@@ -442,10 +452,10 @@ def service_set_international_sms(service_id):
         enabled='on' if 'international_sms' in current_service['permissions'] else 'off'
     )
     if form.validate_on_submit():
-        switch_service_permissions(
+        force_service_permission(
             service_id,
             'international_sms',
-            force=False if form.enabled.data == 'off' else True,
+            on=(form.enabled.data == 'on'),
         )
         return redirect(
             url_for(".service_settings", service_id=service_id)