Earlier, a one-time 'visibility_policy_banner' was displayed to
existing as well as new users to inform them about the new
"follow/unmute topics" feature.
It makes sense to educate only the existing Zulip users about
the new feature using this banner. New users don't need to know
about following topics right away.
This commit makes changes to NOT show the banner to new users.
This will also help to avoid banner overload in the new user
experience.
Fixes#30615.
We immediately navigate the user to the conversation they just
sent a message to if they are not already in the appropriate
conversation view.
This commit adds a first-time banner to explain the same.
Fixes#29575.