Add cancel button to edit of a broadcast message

What does this MR do and why?

Add a cancel button to the broadcast message page for user to go back to the messages index page.

Related to #392886 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Login as Admin
2. Go to admin dashboard (/admin)
3. Click Messages from sidebar
4. Scroll to the bottom and click edit button of an existing message

MR acceptance checklist

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