Simplify column headers (#27557)

app/javascript/mastodon/components/__tests__/button-test.jsx

app/javascript/mastodon/components/column_back_button.tsx

app/javascript/mastodon/components/column_header.jsx

app/javascript/mastodon/features/blocks/index.jsx

app/javascript/mastodon/features/domain_blocks/index.jsx

app/javascript/mastodon/features/follow_requests/index.jsx

app/javascript/mastodon/features/mutes/index.jsx

app/javascript/mastodon/features/pinned_statuses/index.jsx

...mastodon/features/ui/components/__tests__/column-test.jsx

app/javascript/mastodon/features/ui/components/column.jsx

...ascript/mastodon/features/ui/components/column_header.jsx

app/javascript/mastodon/test_helpers.tsx

app/javascript/styles/mastodon/components.scss