Create a Vue app for Broadcast Messages

What does this MR do and why?

Related issue: #368847 (closed)

Adds the starting skeleton for a new Vue app that will replace the current HAML form for broadcast messages. As of right now, the app will only render the list of message IDs to the page. This will eventually be fleshed out to replace all of the existing features

Summary of changes

• Add vue_broadcast_messages feature flag
• Add basic components and store for the new Vue app
• Render the app when the feature is enabled for the current user
• Migrate the existing messages table into a HAML partial to dry up index.html.haml

Screenshots or screen recordings

vue_broadcast_messages enabled

vue_broadcast_messages disabled

How to set up and validate locally

1. Enable the Vue app

   Feature.enable(:vue_broadcast_messages)

2. Navigate to the broadcast messages view: http://gdk.test:3000/admin/broadcast_messages

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.