Add new check for indexing with atomic=True

Briefly, this check helps prevent a long-running index creation from
preventing all reads and writes to a table.

This supplements an existing check
==================================
There is an existing check, `CREATE_INDEX` which warns about creating
indices without the `CONCURRENTLY` argument. It's a helpful check!

However, there are some valid reasons to prefer non-concurrent index
creation. For example, concurrent index creation may struggle to
complete when applied to write-heavy tables (since Postgres will have to
make repeated passes on the table to accommodate the new reads since the
last round).

The danger of `EXCLUSIVE` locks
===============================
If you *do* want to create indices nonconcurrently, there is still a
**huge footgun** to be aware of -- locks which are held in a transaction
are not released until the transaction closes.

Dangerous Django defaults
=========================
By default, any time you add a new Django field with `db_index=True` or
`unique=True`, the automatically generated `Migration` has:

- implicit setting of `atomic=True` (transactions are on)
- an `ALTER TABLE...ADD COLUMN` call (exclusively locks the table)
- a `CREATE INDEX` invocation (*hopefully* quick, due to nullable
  columns)

It's even more dangerous if you combine an `ALTER TABLE` call with
`CREATE INDEX` of an existing, non-nullable column.

Favor `atomic=False` if you must index
======================================
Index-creating migrations should have few `operations`. Since index
creation does not modify row-level data (and only requires locking
against writes), there's no reason to use transactions.

It's prudent to bypass the default `atomic=True` for index creation.

tests/test_project/app_create_index_exclusive/migrations/0001_initial.py

@@ -0,0 +1,30 @@
+# Generated by Django 4.1.7 on 2023-03-09 21:46
+
+from __future__ import annotations
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    initial = True
+
+    dependencies = []
+
+    operations = [
+        migrations.CreateModel(
+            name="User",
+            fields=[
+                (
+                    "id",
+                    models.AutoField(
+                        auto_created=True,
+                        primary_key=True,
+                        serialize=False,
+                        verbose_name="ID",
+                    ),
+                ),
+                ("name", models.TextField()),
+            ],
+        ),
+    ]

tests/test_project/app_create_index_exclusive/migrations/0002_user_email.py

@@ -0,0 +1,20 @@
+# Generated by Django 4.1.7 on 2023-03-09 21:47
+
+from __future__ import annotations
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("app_create_index_exclusive", "0001_initial"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="user",
+            name="email",
+            field=models.EmailField(db_index=True, max_length=254, null=True),
+        ),
+    ]