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

spec/models/sidebar_url_spec.rb

@@ -2,7 +2,11 @@
 
 RSpec.describe SidebarUrl do
   it "validates path" do
-    expect(SidebarUrl.new(name: "categories", value: "/categories").valid?).to eq(true)
-    expect(SidebarUrl.new(name: "categories", value: "/invalid_path").valid?).to eq(false)
+    expect(SidebarUrl.new(icon: "link", name: "categories", value: "/categories").valid?).to eq(
+      true,
+    )
+    expect(SidebarUrl.new(icon: "link", name: "categories", value: "/invalid_path").valid?).to eq(
+      false,
+    )
   end
 end