FEATURE: Add invite link to the sidebar (#29448)

This commit adds a new "Invite" link to the sidebar for all users who can invite to the site. Clicking the link opens the invite modal without changing the current route the user is on. Admins can customize the new link or remove it entirely if they wish by editing the sidebar section.

Internal topic: t/129752.

spec/models/category_spec.rb

@@ -47,13 +47,17 @@ RSpec.describe Category do
 
     it "should delete associated sidebar_section_links when category is destroyed" do
       category_sidebar_section_link = Fabricate(:category_sidebar_section_link)
-      Fabricate(:category_sidebar_section_link, linkable: category_sidebar_section_link.linkable)
-      tag_sidebar_section_link = Fabricate(:tag_sidebar_section_link)
+      category_sidebar_section_link_2 =
+        Fabricate(:category_sidebar_section_link, linkable: category_sidebar_section_link.linkable)
 
       expect { category_sidebar_section_link.linkable.destroy! }.to change {
         SidebarSectionLink.count
       }.from(12).to(10)
-      expect(SidebarSectionLink.last).to eq(tag_sidebar_section_link)
+      expect(
+        SidebarSectionLink.where(
+          id: [category_sidebar_section_link.id, category_sidebar_section_link_2.id],
+        ).count,
+      ).to eq(0)
     end
   end