Improve Collections Header component (#17098)

...lections/components/CollectionHeader/CollectionHeader.jsx

...ns/components/CollectionHeader/CollectionHeader.styled.js

...components/CollectionHeader/CollectionHeader.unit.spec.js

...end/src/metabase/collections/components/Header/Header.jsx

...src/metabase/collections/containers/CollectionContent.jsx

frontend/src/metabase/styled-components/theme/constants.js

frontend/src/metabase/styled-components/theme/index.js

frontend/src/metabase/styled-components/theme/space.js

...d/src/metabase/styled-components/theme/space.unit.spec.js

frontend/test/metabase/collections/util.unit.spec.js