tags: add words column.

Add a `words` column to the tags table. This will be used for parsing
tags into words for word-based matching in autocomplete.

For example, "very_long_hair" can be parsed into ["very", "long", "hair"].

The `array_to_tsvector(words)` index is for performing wildcard
searches. It lets us do e.g

    SELECT * FROM tags WHERE array_to_tsvector(words) @@ 'hand:* & hold:*'

to find tags containing the words "hand*" and "hold*".

db/migrate/20220829184824_add_words_to_tags.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+class AddWordsToTags < ActiveRecord::Migration[7.0]
+  disable_ddl_transaction!
+
+  def change
+    add_column :tags, :words, :string, null: false, array: true, default: [], if_not_exists: true
+    add_index :tags, "array_to_tsvector(words)", using: :gin, if_not_exists: true, algorithm: :concurrently
+  end
+end