compose: Show banner to explain non interleaved view messages fading.

In a non interleaved view when composing a message to another
conversation we fade messages which the user is not replying to,
to reduce the chance they send a message to a recipient they didn't
intend to. Also, it reduces the visual/cognitive processing required
to figure out where their message is going to go.

But, it's not necessarily clear to users that what the
fading means, so this commit adds a one-time compose banner
to explain what's going on the first time this comes up.

Fixes part of #29076.

zerver/models/onboarding_steps.py

@@ -7,7 +7,7 @@ from zerver.models.users import UserProfile
 
 class OnboardingStep(models.Model):
     user = models.ForeignKey(UserProfile, on_delete=CASCADE)
-    onboarding_step = models.CharField(max_length=30)
+    onboarding_step = models.CharField(max_length=40)
     timestamp = models.DateTimeField(default=timezone_now)
 
     class Meta: