modqueue: fix performance regression from including appeals.

* Add index on posts.is_deleted. The modqueue was slow because we the
 appeal condition wasn't constrained to deleted posts, so it degraded to
 a full table scan.

* Avoid extra queries for calculating the page count and disapproval counts.

db/structure.sql

@@ -6757,6 +6757,13 @@ CREATE INDEX index_posts_on_image_height ON public.posts USING btree (image_heig
 CREATE INDEX index_posts_on_image_width ON public.posts USING btree (image_width);
 
 
+--
+-- Name: index_posts_on_is_deleted; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_posts_on_is_deleted ON public.posts USING btree (is_deleted) WHERE (is_deleted = true);
+
+
 --
 -- Name: index_posts_on_is_flagged; Type: INDEX; Schema: public; Owner: -
 --
@@ -7410,6 +7417,8 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20200403210353'),
 ('20200406054838'),
 ('20200427190519'),
-('20200803022359');
+('20200520060951'),
+('20200803022359'),
+('20200816175151');