Resolve edge case from added targeting in broadcast message

Issue to resolve edge case from #409908 (comment 1440139257)

Problem

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.

Solution

1. Require to select at least one role
   • Validation text: "Select at least one role."
2. Add copy to the target path input "Leave blank to target all group and project pages."