Adding Admin > Content Manager > Spaces menu item (#16951)

This menu item will show up once you have added the
`:limit_post_creation_to_admins` feature flag (regardless of whether
it is enabled or not).

If you wish to add the menu item:

```console
rails console runner "FeatureFlag.add(:limit_post_creation_to_admins)"
```

In "adding" the item, it will default to disabled.  However, as an
administrator you should now be able to toggle on and off the
authorization enforcement.

If you wish to remove the menu item:

```console
rails console runner "FeatureFlag.remove(:limit_post_creation_to_admins)"
```

Future plans for this will be to remove the `FeatureFlag.exist?`
conditional so that the menu item shows up.  A major consideration is
that we'll assume that all Forem's have a "default space" in which
folks (by default) can post.

Builds on forem/forem#16897
Closes forem/forem#16842

app/models/admin_menu.rb

@@ -9,6 +9,7 @@ class AdminMenu
     ]
 
     scope :content_manager, "dashboard-line", [
+      item(name: "spaces", controller: "spaces", visible: -> { FeatureFlag.exist?(:limit_post_creation_to_admins) }),
       item(name: "posts", controller: "articles"),
       item(name: "comments", controller: "comments"),
       item(name: "badges", children: [

spec/models/admin_menu_spec.rb

@@ -45,6 +45,28 @@ RSpec.describe AdminMenu do
     it { is_expected.to be_visible }
   end
 
+  describe "scope :content_managers's spaces item" do
+    subject(:spaces) { content_manager.children.detect { |child| child.name == "spaces" } }
+
+    let(:content_manager) { described_class.navigation_items.fetch(:content_manager) }
+
+    context "when the :limit_post_creation_to_admins does not exist" do
+      it { is_expected.not_to be_visible }
+    end
+
+    context "when the :limit_post_creation_to_admins is enabled" do
+      before { FeatureFlag.enable(:limit_post_creation_to_admins) }
+
+      it { is_expected.to be_visible }
+    end
+
+    context "when the :limit_post_creation_to_admins is explicitly disabled" do
+      before { FeatureFlag.disable(:limit_post_creation_to_admins) }
+
+      it { is_expected.to be_visible }
+    end
+  end
+
   describe "scope :customization" do
     subject(:content_manager) { described_class.navigation_items.fetch(:customization) }