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

db/structure.sql

@@ -1896,7 +1896,8 @@ CREATE TABLE public.tags (
     created_at timestamp without time zone NOT NULL,
     updated_at timestamp without time zone NOT NULL,
     is_locked boolean DEFAULT false NOT NULL,
-    is_deprecated boolean DEFAULT false NOT NULL
+    is_deprecated boolean DEFAULT false NOT NULL,
+    words character varying[] DEFAULT '{}'::character varying[] NOT NULL
 );
 
 
@@ -4605,6 +4606,13 @@ CREATE INDEX index_tag_implications_on_consequent_name ON public.tag_implication
 CREATE INDEX index_tag_implications_on_forum_post_id ON public.tag_implications USING btree (forum_post_id);
 
 
+--
+-- Name: index_tags_on_array_to_tsvector_words; Type: INDEX; Schema: public; Owner: -
+--
+
+CREATE INDEX index_tags_on_array_to_tsvector_words ON public.tags USING gin (array_to_tsvector((words)::text[]));
+
+
 --
 -- Name: index_tags_on_is_deprecated; Type: INDEX; Schema: public; Owner: -
 --
@@ -5976,6 +5984,7 @@ INSERT INTO "schema_migrations" (version) VALUES
 ('20220514175125'),
 ('20220525214746'),
 ('20220623052547'),
-('20220627211714');
+('20220627211714'),
+('20220829184824');