refactoring: move sql files

resources/sql/functions/follow/follow.sql

@@ -0,0 +1,23 @@
+create or replace function follow(reference regclass, _target_id uuid, _citizen_id uuid) returns void
+    language plpgsql as
+$$
+begin
+    if reference = 'article'::regclass then
+        insert into follow_article (citizen_id, target_id)
+        values (_citizen_id, _target_id)
+        on conflict (citizen_id, target_id) do nothing;
+    elseif reference = 'constitution'::regclass then
+        insert into follow_constitution (citizen_id, target_id)
+        values (_citizen_id, _target_id)
+        on conflict (citizen_id, target_id) do nothing;
+    elseif reference = 'citizen'::regclass then
+        insert into follow_citizen (citizen_id, target_id)
+        values (_citizen_id, _target_id)
+        on conflict (citizen_id, target_id) do nothing;
+    else
+        raise exception '% no implemented', reference::text;
+    end if;
+end;
+$$;
+
+-- drop function if exists follow(regclass, uuid, uuid);