posts: add index on rating column.

Should improve performance for rating:e and rating:q searches. Rating:s
isn't isn't indexed because Postgres is unlikely to use the index for
rating:s searches (the selectivity is too low, ~77% of all posts are
rating:s).

db/migrate/20210127012303_add_rating_index_to_posts.rb

@@ -0,0 +1,5 @@
+class AddRatingIndexToPosts < ActiveRecord::Migration[6.1]
+  def change
+    add_index :posts, :rating, where: "rating != 's'"
+  end
+end