Adding missing indexes on taggings table

API requests were timing out because `tag_id` was missing an index. These
migrations were imported by running https://github.com/mbleigh/acts-as-taggable-on#post-installation:

```
bundle exec rake acts_as_taggable_on_engine:install:migrations
```

It looks like the `acts-as-tagglable` gem added the indexes in v4.0.0, but
when we upgraded from v3.5.0 (back in 2016 via
f571aeb5) we did not add them.

On staging, there are about 10.7 million rows on the `taggings` table. It took about 30 seconds for each index to be created.

On production, there are about 17.4 million rows, so I suspect the time to be about a minute per index.

Closes #43927

changelogs/unreleased/sh-add-missing-acts-as-taggable-indices.yml

+---
+title: Adding missing indexes on taggings table
+merge_request:
+author:
+type: performance

db/migrate/20180306134842_add_missing_indexes_acts_as_taggable_on_engine.rb

+# This migration comes from acts_as_taggable_on_engine (originally 6)
+#
+# It has been modified to handle no-downtime GitLab migrations. Several
+# indexes have been removed since they are not needed for GitLab.
+class AddMissingIndexesActsAsTaggableOnEngine < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :taggings, :tag_id unless index_exists? :taggings, :tag_id
+    add_concurrent_index :taggings, [:taggable_id, :taggable_type] unless index_exists? :taggings, [:taggable_id, :taggable_type]
+  end
+
+  def down
+    remove_concurrent_index :taggings, :tag_id
+    remove_concurrent_index :taggings, [:taggable_id, :taggable_type]
+  end
+end

db/schema.rb

@@ -1733,7 +1733,9 @@ ActiveRecord::Schema.define(version: 20180307012445) do
   end
 
   add_index "taggings", ["tag_id", "taggable_id", "taggable_type", "context", "tagger_id", "tagger_type"], name: "taggings_idx", unique: true, using: :btree
+  add_index "taggings", ["tag_id"], name: "index_taggings_on_tag_id", using: :btree
   add_index "taggings", ["taggable_id", "taggable_type", "context"], name: "index_taggings_on_taggable_id_and_taggable_type_and_context", using: :btree
+  add_index "taggings", ["taggable_id", "taggable_type"], name: "index_taggings_on_taggable_id_and_taggable_type", using: :btree
 
   create_table "tags", force: :cascade do |t|
     t.string "name"