Fix #5094: db/populate.rb is broken.

Rewrite db/populate.rb:

* Fix broken code.
* Pull random posts from Danbooru for more realistic data.
* Generate more data (wiki pages, artist commentaries, artist urls).
* Make the amount of data generated configurable with environment variables.
* Use FFaker to generate better random text and usernames.

Usage:

* docker-compose exec danbooru bin/rails runner db/populate.rb # with Docker
* bin/rails runner db/populate.rb                              # without Docker

Gemfile

@@ -56,6 +56,7 @@ gem "crass"
 gem "public_suffix"
 gem "elastic-apm"
 gem "debug"
+gem "ffaker"
 
 group :development do
   gem 'rubocop', require: false
@@ -78,7 +79,6 @@ group :test do
   gem "shoulda-matchers"
   gem "factory_bot"
   gem "mocha", require: "mocha/minitest"
-  gem "ffaker"
   gem "simplecov", require: false
   gem "minitest-ci"
   gem "minitest-reporters", require: "minitest/reporters"