This is part several updates for #22524.
Testing note: I removed the tests test_compose_invite_users_clicked
and test_compose_invite_close_clicked, since they heavily relied on
the old way of rendering banners and were too UI-focused (instead of
logic focused) for me to feel like it was worth testing that the
banners removed when clicking the buttons.
Error banners were rendered in a different place than warnings.
Now compose errors and warnings share the same template and styling
in compose_banner.hbs
Testing notes:
* I removed test_message_overflow since it seemed not to test
anything that test_check_overflow_text wasn't already testing.
* private_message_recipient() can't be called to set emails to non-valid
emails, so `invalid_recipient` cannot be tested (and is likely very
difficult to trigger in production, if possible at all).
This is the beginning of a fix for #22524 which converts several
banners to a new style. As a part of that set of changes, this
commit creates the shared template and warning styling. The
resolved topic warning was picked (for no particular reason)
to migrate first. Further commits updating other banners
to follow.
evykassirer
Anders Kaseorg
Aman Agrawal
Priyank Patel