Optimize reactions table (#13406)

* Optimize reactions table

Change reactions primary key to (PostId, UserId, EmojiName) so fetching
reactions for post will use primary key lookup instead of table scan.

* fix db version

* review fixes

* update database schema in scripts/

scripts/mattermost-postgresql-5.0.sql

@@ -1080,7 +1080,7 @@ ALTER TABLE ONLY public.preferences
 --
 
 ALTER TABLE ONLY public.reactions
-    ADD CONSTRAINT reactions_pkey PRIMARY KEY (userid, postid, emojiname);
+    ADD CONSTRAINT reactions_pkey PRIMARY KEY (postid, userid, emojiname);
 
 
 --