[Accessibility] Get rid of ALLCAPS headings and buttons in the default Decidim styling

ALLCAPS headings/buttons are bad for accessibility: - They make the text less readable - They are problematic for dyslexic people - Some screen readers read them letter by letter, even if the capitalization has been made using CSS This kind of styling is not particularly forbidden by the WCAG 2.1 guidelines but makes the system less accessible e.g. for the mentioned reasons, so it should not be encouraged by Decidim's default styling. Further reading: https://github.com/humanmade/hm-pattern-library/issues/75 https://accessibility.huit.harvard.edu/design-readability Also, why are ALLCAPS forbidden in the proposal titles by the etiquette validator if they are enforced in Decidim's own headings? ;)