Add onboarding prompt when home feed too slow in web UI (#25267)

app/javascript/images/friends-cropped.png

...javascript/mastodon/features/community_timeline/index.jsx

app/javascript/mastodon/features/explore/links.jsx

app/javascript/mastodon/features/explore/statuses.jsx

app/javascript/mastodon/features/explore/tags.jsx

...odon/features/home_timeline/components/explore_prompt.jsx

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

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

app/javascript/mastodon/locales/en.json

app/javascript/styles/mastodon-light/diff.scss

app/javascript/styles/mastodon/components.scss