FIX: my links in sidebar section (#20754)

Links like `/my/preferences` were invalid in custom section. The reason is that `/my` links are just redirects from backend, and they are not recognized as valid Ember paths.

https://github.com/discourse/discourse/blob/main/config/routes.rb#L433

Therefore, regex match allowlist was added - similar to backend check:

https://github.com/discourse/discourse/blob/main/app/controllers/users_controller.rb#L471

/safe-mode is same case

spec/requests/sidebar_sections_controller_spec.rb

@@ -61,6 +61,7 @@ RSpec.describe SidebarSectionsController do
                },
                { icon: "address-book", name: "tags", value: "/tags" },
                { icon: "external-link-alt", name: "Discourse", value: "https://discourse.org" },
+               { icon: "external-link-alt", name: "My preferences", value: "/my/preferences" },
              ],
            }
 
@@ -73,7 +74,7 @@ RSpec.describe SidebarSectionsController do
       expect(sidebar_section.user).to eq(user)
       expect(sidebar_section.public).to be false
       expect(UserHistory.count).to eq(0)
-      expect(sidebar_section.sidebar_urls.count).to eq(3)
+      expect(sidebar_section.sidebar_urls.count).to eq(4)
       expect(sidebar_section.sidebar_urls.first.icon).to eq("link")
       expect(sidebar_section.sidebar_urls.first.name).to eq("categories")
       expect(sidebar_section.sidebar_urls.first.value).to eq("/categories")
@@ -86,6 +87,10 @@ RSpec.describe SidebarSectionsController do
       expect(sidebar_section.sidebar_urls.third.name).to eq("Discourse")
       expect(sidebar_section.sidebar_urls.third.value).to eq("https://discourse.org")
       expect(sidebar_section.sidebar_urls.third.external).to be true
+      expect(sidebar_section.sidebar_urls.fourth.icon).to eq("external-link-alt")
+      expect(sidebar_section.sidebar_urls.fourth.name).to eq("My preferences")
+      expect(sidebar_section.sidebar_urls.fourth.value).to eq("/my/preferences")
+      expect(sidebar_section.sidebar_urls.fourth.external).to be false
     end
 
     it "does not allow regular user to create public section" do

spec/system/custom_sidebar_sections_spec.rb

@@ -34,6 +34,25 @@ describe "Custom sidebar sections", type: :system, js: true do
     expect(sidebar).to have_link("Sidebar Tags")
   end
 
+  it "allows the user to create custom section with /my link" do
+    visit("/latest")
+    sidebar.open_new_custom_section
+
+    expect(section_modal).to be_visible
+    expect(section_modal).to have_disabled_save
+    expect(sidebar.custom_section_modal_title).to have_content("Add custom section")
+
+    section_modal.fill_name("My section")
+
+    section_modal.fill_link("My preferences", "/my/preferences")
+    expect(section_modal).to have_enabled_save
+
+    section_modal.save
+
+    expect(page).to have_button("My section")
+    expect(sidebar).to have_link("My preferences")
+  end
+
   it "allows the user to create custom section with external link" do
     visit("/latest")
     sidebar.open_new_custom_section