FEATURE: Initial themes config area with grid (#28828)

* UX: More additions

* UX: more

* DEV: Add admin/config/themes route

* UX: Use admin config card

* syntax merge fixes

* cleanup

* cleanup

* checkbox

* more

* error

* save on click

* more

* fix setter

* DEV: Implement vanilla checkbox

* cleanup

* UX: save themes as default

* DEV: Add component list to card

* DEV: Add placeholder for no screenshots

* DEV: Fix default theme reactivity

Also add content/optionalAction yields to config area
card and put the theme user selectable checkbox there,
along with adding styles.

* DEV: Change to generic "look and feel" config area

* DEV: Auto redirect to themes on base look and feel route

* UX: Remove computed from sorted themes

* linting

* UX: Turn update icon into button that routes to settings

* DEV: remove unused function

* UX: center icons with title

* DEV: Lint

* UX: Hook up theme preview button

* DEV: Minor fixes

---------

Co-authored-by: Martin Brennan <martin@discourse.org>
Co-authored-by: Joffrey JAFFEUX <j.jaffeux@gmail.com>

app/controllers/admin/config/look_and_feel_controller.rb

@@ -0,0 +1,6 @@
+# frozen_string_literal: true
+
+class Admin::Config::LookAndFeelController < Admin::AdminController
+  def themes
+  end
+end