This is part several updates for #22524.
This commit also removes `compose_error.hide()` since there is only
one remaining use of it (which has been replaced by the contents of
`hide()`). Eventually `#compose-send-status` should be replaced
fully in a future PR, after we rewrite the upload banner to use
the new banner template.
Testing note: I removed the test test_compose_not_subscribed_clicked
since it heavily relies on the old way of rendering banners and is too
UI-focused (instead of logic focused) for me to feel like it's worth
testing that the banner is removed when clicking the button.
evykassirer
Anders Kaseorg
Steve Howell
YashRE42