Improve alert banner

[MisRob]

Summary

Studio uses multiple styles for error banners (with black text, with red text, ...). We've discussed with @tomiwaoLE which one to choose for StudioBanner component that gradually replaces Vuetify-based banners and will ultimately become the only alert component. This PR aligns StudioBanner with the decisions taken and this design:

As of now, changes affect these places:

	Before	After
'Make public' dialog in channels administration
Storage form request in settings

but will gradually take effect in the whole Studio.

[nucleogenesis]

Code changes look good to me & the screenshot agrees with the guidance. Looks so much better too!

[MisRob]

Thanks @nucleogenesis