Mainly, added the dwellings table, implied by the revised ERD.

Some other tidy up.

resources/migrations/20180526162051-dwellings.down.sql

@@ -0,0 +1,8 @@
+alter table electors
+  add column address_id references addresses on delete no action;
+
+update electors
+  set address_id =
+    (select address_id
+       from dwellings
+       where dwellings.id electors.dwelling_id);

resources/migrations/20180526162051-dwellings.up.sql

@@ -0,0 +1,11 @@
+CREATE TABLE  dwellings
+(
+  id INT NOT NULL PRIMARY KEY,
+  address_id INT NOT NULL references addresses on delete no action,
+  sub_address VARCHAR( 32)
+);
+
+alter table electors
+  add column dwelling_id int references dwellings on delete no action;
+
+alter table electors drop column address_id;