Adding possibly missing indices (#16323)

This is meant as a conversation.  Throughout the code-base we have have
`Tag.order(hotness_score: :desc)` and with PR #16322 we'll also have
`Tag.order(taggings_count: :desc)`.

My understanding is that if we have sorting, we might want to consider
an index.  I put this forward as a conversation with a possible quick win.

Related to #16322

db/migrate/20220126205052_adding_indices_to_tag.rb

@@ -0,0 +1,7 @@
+class AddingIndicesToTag < ActiveRecord::Migration[6.1]
+  disable_ddl_transaction!
+  def change
+    add_index :tags, :hotness_score, algorithm: :concurrently
+    add_index :tags, :taggings_count, algorithm: :concurrently
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2021_12_22_040359) do
+ActiveRecord::Schema.define(version: 2022_01_26_205052) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "citext"
@@ -1121,9 +1121,11 @@ ActiveRecord::Schema.define(version: 2021_12_22_040359) do
     t.datetime "updated_at", null: false
     t.text "wiki_body_html"
     t.text "wiki_body_markdown"
+    t.index ["hotness_score"], name: "index_tags_on_hotness_score"
     t.index ["name"], name: "index_tags_on_name", unique: true
     t.index ["social_preview_template"], name: "index_tags_on_social_preview_template"
     t.index ["supported"], name: "index_tags_on_supported"
+    t.index ["taggings_count"], name: "index_tags_on_taggings_count"
   end
 
   create_table "tweets", force: :cascade do |t|