Resolve UI edge case from added targeting in broadcast message

What does this MR do and why?

On broadcast message create form, if the user picks Show only to users who have specific roles on groups/project pages but leave the target roles and target path empty, the message will actually show to all users on all pages, not just groups/project page. This may be confusing to user.

So we want to add validation to broadcast message target roles selection to require at least one role being selected and additional copy to emphasize when targeting roles, the target path is scoped to group and project pages, not all pages.

Screenshots or screen recordings

Before
After
After (tries to submit without selecting role)

How to set up and validate locally

1. Sign in as admin
2. In sidebar, go to admin dashboard by clicking Your work > Admin Area
3. On admin dashboard, in sidebar, click Messages
4. On broadcast message page, click Add new message
5. Select Show only to users who have specific roles on groups/project pages in the Target broadcast message dropdown.

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.

Related to #417032 (closed)