Complete admin page with sidebar navigation and update functionality

What does this MR do and why?

Resolves #511014 (closed)

1. adds navigation to the admin sidebar
2. adds edit, update and corresponding views to targeted messages

References

Screenshots or screen recordings

Index page with edit buttons	Edit page

How to set up and validate locally

This feature is EE-only for now.

1. Enable the feature in rails console:

Feature.enable(:targeted_messages_admin_ui)

2. Sign in as admin
3. Go to /admin, click the Targeted messages navigation in the sidebar
4. Click Add new message navigate to /admin/targeted_messages/new for creating a new targeted message.
5. Click the pencil icon button to navigate to edit page for updating targeted message.

MR acceptance checklist

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