Add pending submissions pill on the navbar

What does this MR do and why?

Related to #593 (closed)

Adds a small pill indicator in the navbar, visible only to admins, showing the current count of pending submissions and linking to /admin.

It's hidden entirely when the count is 0.

Screenshots or screen recordings

Before	After

Validation steps

1. First, give yourself admin access to be able to see the pill

   rake data:admin[YOUR_USERNAME]

2. Then open a Rails console and create a fake submission :

      user = User.first || FactoryBot.create(:user, username: 'testuser')
   
      FactoryBot.create(
        :activity,
        platform: :contributor_platform,
        kind: :contributor_platform_content,
        user_id: user.id,
        metadata: { 'name' => 'Test submission', 'link' => 'https://example.com', 'status' => 'pending' }
      )

3. Reload the page

4. Click the pill and confirm it navigates to /admin

/cc @leetickett-gitlab