Add media timeline (#6631)

app/javascript/mastodon/actions/streaming.js

app/javascript/mastodon/actions/timelines.js

app/javascript/mastodon/features/community_timeline/index.js

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

app/javascript/mastodon/features/ui/index.js

app/javascript/mastodon/locales/ar.json

app/javascript/mastodon/locales/bg.json

app/javascript/mastodon/locales/ca.json

app/javascript/mastodon/locales/co.json

app/javascript/mastodon/locales/de.json

app/javascript/mastodon/locales/defaultMessages.json

app/javascript/mastodon/locales/el.json

app/javascript/mastodon/locales/en.json

app/javascript/mastodon/locales/eo.json

app/javascript/mastodon/locales/es.json

app/javascript/mastodon/locales/eu.json

app/javascript/mastodon/locales/fa.json

app/javascript/mastodon/locales/fi.json

app/javascript/mastodon/locales/fr.json

app/javascript/mastodon/locales/gl.json