Show new "Your work" sidebar on Activity and Milestones dashboards

What does this MR do and why?

This builds on !107345 (merged), adding two more menu items to the new Your work sidebar (#384342 - closed):

• Milestones
• Activity

Screenshots or screen recordings

How to set up and validate locally

• Make sure you have the your_work_sidebar feature flag enabled:

Feature.enable(:your_work_sidebar)

• While logged in, go to http://gdk.test:3000/dashboard/projects. This page should show the sidebar now and the Projects item should show as active.
• Navigate to "Milestones" from within the sidebar. The milestones dashboard should show and the sidebar menu item you've just clicked should now show as active.
• Do the same for "Activity".

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

lib/sidebars/your_work/menus/activity_menu.rb

+# frozen_string_literal: true
+
+module Sidebars
+  module YourWork
+    module Menus
+      class ActivityMenu < ::Sidebars::Menu
+        override :link
+        def link
+          activity_dashboard_path
+        end
+
+        override :title
+        def title
+          _('Activity')
+        end
+
+        override :sprite_icon
+        def sprite_icon
+          'history'
+        end
+
+        override :render?
+        def render?
+          !!context.current_user
+        end
+
+        override :active_routes
+        def active_routes
+          { path: 'dashboard#activity' }
+        end
+      end
+    end
+  end
+end