Make (pre|post)_migrate scripts for the index table only if working on 'transient'.
Make (pre|post)_migrate scripts for the index table only if working on 'transient'.

Django run (pre|post)_migrate script once per database.
As we have two databases, the create_index is launch twice with different
kwargs['using'] ('default' and 'transient'). We should try to create
the index table only when we are working on the transient database.

Most of the time, this is not important and create a new index table
twice is not important.
However, if we run tests, the database are configured and migrate
one after the other and the 'transient' database may be miss-configured
at a time. By creating the table only at the right time, we ensure that
everything is properly configured.

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