refactoring #19: rename column workgroup to workgroup_id

resources/sql/migrations/0000-init_schema.up.sql

@@ -37,10 +37,10 @@ create table workgroup
 
 create table citizen_in_workgroup
 (
-    citizen_id uuid                      not null references citizen (id),
-    workgroup  uuid                      not null references workgroup (id),
-    created_at timestamptz default now() not null,
-    primary key (citizen_id, workgroup)
+    citizen_id   uuid                      not null references citizen (id),
+    workgroup_id uuid                      not null references workgroup (id),
+    created_at   timestamptz default now() not null,
+    primary key (citizen_id, workgroup_id)
 );
 
 create table moderator