Make media_gallery.toggle_visible less confusing to translate (#15993)

app/javascript/mastodon/components/media_gallery.js

app/javascript/mastodon/locales/defaultMessages.json

app/javascript/mastodon/locales/en.json

app/javascript/mastodon/locales/hi.json

app/javascript/mastodon/locales/kn.json

app/javascript/mastodon/locales/lt.json

app/javascript/mastodon/locales/lv.json

app/javascript/mastodon/locales/mk.json

app/javascript/mastodon/locales/ml.json

app/javascript/mastodon/locales/mr.json

app/javascript/mastodon/locales/ms.json

app/javascript/mastodon/locales/sa.json

app/javascript/mastodon/locales/szl.json

app/javascript/mastodon/locales/tai.json

app/javascript/mastodon/locales/tt.json

app/javascript/mastodon/locales/ug.json

app/javascript/mastodon/locales/ur.json