Put the less-specific column on the left-hand side (#193)

Some database systems can make better use of an index if the index
columns are ordered left-to-right in increasing specificity.  I'm not
sure if Postgres is one of those databases, but it also makes sense to
me to read an index as "ordered by type and then ID".

db/migrate/20180118192721_create_glitch_notes.rb

@@ -4,7 +4,7 @@ class CreateGlitchNotes < ActiveRecord::Migration[5.1]
       t.bigint :target_id, null: false
       t.string :target_type, null: false
       t.text :note, null: false
-      t.index [:target_id, :target_type]
+      t.index [:target_type, :target_id]
       t.timestamps
     end
   end

db/schema.rb

@@ -188,7 +188,7 @@ ActiveRecord::Schema.define(version: 20180118192721) do
     t.text "note", null: false
     t.datetime "created_at", null: false
     t.datetime "updated_at", null: false
-    t.index ["target_id", "target_type"], name: "index_glitch_notes_on_target_id_and_target_type"
+    t.index ["target_type", "target_id"], name: "index_glitch_notes_on_target_type_and_target_id"
   end
 
   create_table "imports", force: :cascade do |t|