refactor: wp-280 proper message mixin

[wesleyboar]

Overview / Changes

Create/Use a standalone message mixin.

Related

• WP-280

Testing

1. Regression test "C Message" component.
2. Regression test "Admonition" component.
3. Regression test "Form" component messages.

UI

c-message

	standard	portal
top
end

c-form & s-form & Django CMS Forms

The s-form and Django CMS Forms are not pictured, but they match.

cms	portal

admonition

	standard
top
end