DEV: endpoint to reset community community-section (#21664)

In upcoming PRs, admins will be able to edit the Community section. We need an endpoint which allows resetting it to the default state.

spec/models/sidebar_section_spec.rb

@@ -3,10 +3,25 @@
 RSpec.describe SidebarSection do
   fab!(:user) { Fabricate(:user) }
   fab!(:sidebar_section) { Fabricate(:sidebar_section, user: user) }
+  let(:community_section) do
+    SidebarSection.find_by(section_type: SidebarSection.section_types[:community])
+  end
 
   it "uses system user for public sections" do
     expect(sidebar_section.user_id).to eq(user.id)
     sidebar_section.update!(public: true)
     expect(sidebar_section.user_id).to eq(Discourse.system_user.id)
   end
+
+  it "resets Community section to the default state" do
+    community_section.update!(title: "test")
+    community_section.sidebar_section_links.first.linkable.update!(name: "everything edited")
+    community_section.sidebar_section_links.last.destroy!
+
+    community_section.reset_community!
+    expect(community_section.reload.title).to eq("Community")
+    expect(community_section.sidebar_section_links.all.map { |link| link.linkable.name }).to eq(
+      ["Everything", "My Posts", "Review", "Admin", "Users", "About", "FAQ", "Groups", "Badges"],
+    )
+  end
 end