Migrate to GlBroadcastMessage in app/views/admin/broadcast_messages/_form.html.haml

Update app/views/admin/broadcast_messages/_form.html.haml to leverage the GlBroadcastMessage Pajamas component.

Refer to the epic for detailed migration instructions.

• The alert is rendered with render Pajamas::AlertComponent.new.
• The alert's body is placed in the component's block and wrapped in a .gl-alert-body selector.
• If the alert has actions, they are rendered in the component's block, below the body, and wrapped in a .gl-alert-actions selector.
  • If there are multiple action buttons, they are separated with a .gl-ml-3 margin or equivalent.
• If the alert has a title, it is set via the title parameter.
• The alert's variant is set via the variant parameter (can be omitted for info alerts).
• If the alert is non-dismissible, the dismissible parameter is set to false.
• Alert's classes are set via the alert_class parameter.
• Alert's data attributes are set via the alert_data parameter.
• Alert's dismiss button's classes are set via the close_button_class parameter.
• Alert's dismiss button's data attributes are set via the close_button_data parameter.