[ADD] newsletter section

...source/_patterns/atoms/form-fields/form-checkbox.mustache

...de/source/_patterns/atoms/form-fields/form-input.mustache

...e/source/_patterns/atoms/form-fields/form-select.mustache

...atterns/molecules/articles/small-article-preview.mustache

...de/source/_patterns/organisms/newsletter-section.mustache

...eguide/source/_patterns/organisms/region-section.mustache

main/styleguide/source/_patterns/templates/homepage.mustache

main/styleguide/source/css/atoms/form-field.pcss

main/styleguide/source/css/style.pcss