Change defaultMessage for spoiler placeholder (#4329)

...ript/mastodon/features/compose/components/compose_form.js

app/javascript/mastodon/locales/ar.json

app/javascript/mastodon/locales/bg.json

app/javascript/mastodon/locales/ca.json

app/javascript/mastodon/locales/de.json

app/javascript/mastodon/locales/defaultMessages.json

app/javascript/mastodon/locales/en.json

app/javascript/mastodon/locales/eo.json

app/javascript/mastodon/locales/es.json

app/javascript/mastodon/locales/fa.json

app/javascript/mastodon/locales/fi.json

app/javascript/mastodon/locales/fr.json

app/javascript/mastodon/locales/he.json

app/javascript/mastodon/locales/hr.json

app/javascript/mastodon/locales/hu.json

app/javascript/mastodon/locales/id.json

app/javascript/mastodon/locales/io.json

app/javascript/mastodon/locales/it.json

app/javascript/mastodon/locales/ja.json

app/javascript/mastodon/locales/ko.json