FIX: Ensure post_hotlinked_media index does not exceed size limit (#16609)

On some installations, this would fail with 'index row size exceeds btree version 4 maximum'. This commit replaces the (post_id, url)` index with a `(post_id, md5(url))` index, which is much more space efficient.

db/migrate/20220428094026_create_post_hotlinked_media.rb

@@ -22,9 +22,15 @@ class CreatePostHotlinkedMedia < ActiveRecord::Migration[6.1]
       t.bigint :upload_id
       t.timestamps
 
-      t.index [:post_id, :url], unique: true
+      # Failed on some installations due to index size. Repaired in 20220428094027
+      # t.index [:post_id, :url], unique: true
     end
 
+    execute <<~SQL
+      CREATE UNIQUE INDEX index_post_hotlinked_media_on_post_id_and_url_md5
+      ON post_hotlinked_media (post_id, md5(url));
+    SQL
+
     reversible do |dir|
       dir.up do
         execute <<~SQL