Add Rails/AddColumnIndex rubocop rule

The `column` method in ActiveRecord::ConnectionAdapters::TableDefinition
supports adding the `index:` option. The documentation says:

> Instantiates a new column for the table. See connection.add_column for
> available options.
>
> Additional options are:
>
> :index - Create an index for the column. Can be either true or an
> options hash.

So basically the `connection.add_column` method silently ignores the
`index:` option, and whenever we intended to create an index this way,
we didn't.

We're creating a new migration where we properly add the indexes that
weren't added when we intended to.

Thanks to the rubocop-rails team, who added this cop in version 2.11.0
and helped us notice this bug.

.rubocop.yml

@@ -208,6 +208,9 @@ Performance/Sum:
 Rails/ActiveRecordCallbacksOrder:
   Enabled: true
 
+Rails/AddColumnIndex:
+  Enabled: true
+
 Rails/CreateTableWithTimestamps:
   Enabled: true
   Exclude: