FIX: improvements for user custom sections (#20190)

Improvements for this PR: https://github.com/discourse/discourse/pull/20057

What was fixed:
- [x] Use ember transitions instead of full reload
- [x] Link was inaccurately kept active
- [x] "+ save" renamed to just "save"
- [x] Render emojis in link name
- [x] UI to set icon
- [x] Delete link is trash icon instead of "x"
- [x] Add another link to on the left and rewording
- [x] Raname "link name" -> "name", "points to" ->  link
- [x] Add limits to fields
- [x] Move add section button to the bottom

app/controllers/sidebar_sections_controller.rb

@@ -42,7 +42,7 @@ class SidebarSectionsController < ApplicationController
   end
 
   def links_params
-    params.permit(links: %i[name value id _destroy])["links"]
+    params.permit(links: %i[icon name value id _destroy])["links"]
   end
 
   def check_if_member_of_group