models: Rename MutedTopic to UserTopic.

Part of #19272

We still keep refering to this model with "MutedTopic" to reduce the
diff size of this commit. The alias will be removed in the next commit.

This commit skips on renaming the `date_muted` field to something more
general. That will be done in further commits, along with the code and
API changes.

zerver/migrations/0340_rename_mutedtopic_to_usertopic.py

@@ -0,0 +1,21 @@
+# Generated by Django 3.2.5 on 2021-07-23 13:15
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("zerver", "0339_remove_realm_add_emoji_by_admins_only"),
+    ]
+
+    operations = [
+        migrations.RenameModel(
+            old_name="MutedTopic",
+            new_name="UserTopic",
+        ),
+        migrations.AlterModelTable(
+            name="usertopic",
+            table="zerver_mutedtopic",
+        ),
+    ]

zerver/models.py

@@ -2128,7 +2128,7 @@ post_save.connect(flush_stream, sender=Stream)
 post_delete.connect(flush_stream, sender=Stream)
 
 
-class MutedTopic(models.Model):
+class UserTopic(models.Model):
     id: int = models.AutoField(auto_created=True, primary_key=True, verbose_name="ID")
     user_profile: UserProfile = models.ForeignKey(UserProfile, on_delete=CASCADE)
     stream: Stream = models.ForeignKey(Stream, on_delete=CASCADE)
@@ -2144,8 +2144,16 @@ class MutedTopic(models.Model):
     class Meta:
         unique_together = ("user_profile", "stream", "topic_name")
 
+        # This model was originally called "MutedTopic". We
+        # generalized it to "UserTopic", but have not yet done the
+        # database migration to rename the table and indexes.
+        db_table = "zerver_mutedtopic"
+
     def __str__(self) -> str:
-        return f"<MutedTopic: ({self.user_profile.email}, {self.stream.name}, {self.topic_name}, {self.date_muted})>"
+        return f"<UserTopic: ({self.user_profile.email}, {self.stream.name}, {self.topic_name}, {self.date_muted})>"
+
+
+MutedTopic = UserTopic
 
 
 class MutedUser(models.Model):