Redirect root path requests to the tenants administration

When the `multitenancy_management_mode` is enabled.

In order to avoid infinite redirects when regular users try to access
the admin section, we're redirecting to the account page in this case.
Otherwise, the admin section would redirect to the root path, which
would redirect to the admin section, which would redirect to the root
path, and so on.

app/controllers/concerns/access_denied_handler.rb

@@ -4,7 +4,13 @@ module AccessDeniedHandler
   included do
     rescue_from CanCan::AccessDenied do |exception|
       respond_to do |format|
-        format.html { redirect_to main_app.root_path, alert: exception.message }
+        format.html do
+          if Rails.application.multitenancy_management_mode?
+            redirect_to main_app.account_path, alert: exception.message
+          else
+            redirect_to main_app.root_path, alert: exception.message
+          end
+        end
         format.json { render json: { error: exception.message }, status: :forbidden }
       end
     end

config/routes.rb

@@ -8,6 +8,10 @@ Rails.application.routes.draw do
   draw :admin
   draw :devise
 
+  constraints lambda { |request| Rails.application.multitenancy_management_mode? } do
+    get "/", to: "admin/tenants#index"
+  end
+
   constraints lambda { |request| !Rails.application.multitenancy_management_mode? } do
     draw :budget
     draw :comment

spec/system/multitenancy_management_mode_spec.rb

@@ -25,4 +25,34 @@ describe "Multitenancy management mode", :admin do
       expect(page).to have_css "li", count: 2
     end
   end
+
+  scenario "redirects root path requests to the admin tenants path" do
+    visit root_path
+
+    expect(page).to have_content "CONSUL ADMINISTRATION", normalize_ws: true
+    expect(page).to have_content "Multitenancy"
+    expect(page).not_to have_content "Most active proposals"
+  end
+
+  scenario "does not redirect other tenants when visiting the root path", :seed_tenants do
+    create(:tenant, schema: "mars")
+
+    with_subdomain("mars") do
+      visit root_path
+
+      expect(page).to have_content "Most active proposals"
+      expect(page).not_to have_content "Multitenancy"
+      expect(page).not_to have_content "CONSUL ADMINISTRATION", normalize_ws: true
+    end
+  end
+
+  scenario "redirects to account path when regular users try to access the admin section" do
+    logout
+    login_as(create(:user))
+
+    visit admin_root_path
+
+    expect(page).to have_current_path account_path
+    expect(page).to have_content "You do not have permission to access this page."
+  end
 end