FEATURE: Allow theme tests to be run in production (take 2) (#12845)

This commit allows site admins to run theme tests in production via a new `/theme-qunit` route. When you visit `/theme-qunit`, you'll see a list of the themes/components installed on your site that have tests, and from there you can select a theme or component that you run its tests.

We also have a new rake task `themes:install_and_test` that can be used to install a list of themes/components on a temporary database and run the tests of the themes/components that are installed. This rake task can be useful when upgrading/deploying a Discourse instance to make sure that the installed themes/components are compatible with the new Discourse version being deployed, and if the tests fail you can abort the build/deploy process so you don't end up with a broken site.

lib/tasks/db.rake

@@ -329,26 +329,7 @@ task 'db:validate_indexes', [:arg] => ['db:ensure_post_migrations', 'environment
 
   db = TemporaryDb.new
   db.start
-
-  ActiveRecord::Base.establish_connection(
-    adapter: 'postgresql',
-    database: 'discourse',
-    port: db.pg_port,
-    host: 'localhost'
-  )
-
-  puts "Running migrations on blank database!"
-
-  old_stdout = $stdout.clone
-  old_stderr = $stderr.clone
-  $stdout.reopen(File.new('/dev/null', 'w'))
-  $stderr.reopen(File.new('/dev/null', 'w'))
-
-  SeedFu.quiet = true
-  Rake::Task["db:migrate"].invoke
-
-  $stdout.reopen(old_stdout)
-  $stderr.reopen(old_stderr)
+  db.migrate
 
   ActiveRecord::Base.establish_connection(
     adapter: 'postgresql',