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.

docs/incompatibilities.md

@@ -40,6 +40,7 @@ You can ignore checks through the `--exclude-migration-tests` option by specifyi
 | `RUNPYTHON_MODEL_VARIABLE_NAME`    | The model variable name is different from the model class itself                                                     | Warning      |
 | `RUNSQL_REVERSIBLE`                | RunSQL data migration is not reversible (missing reverse SQL)                                                        | Warning      |
 | `CREATE_INDEX`                     | (Postgresql specific) Creating an index without the concurrent keyword will lock the table and may generate downtime | Warning      |
+| `CREATE_INDEX_EXCLUSIVE`           | (Postgresql specific) Creating an index in a transaction (after an `EXCLUSIVE` lock) prolongs the exclusive lock     | Error        |
 | `DROP_INDEX`                       | (Postgresql specific) Dropping an index without the concurrent keyword will lock the table and may generate downtime | Warning      |
 | `REINDEX`                          | (Postgresql specific) Reindexing will lock the table and may generate downtime                                       | Warning      |