Detaching a partition from a partitioned table that's constrained by
foreign keys requires additional action triggers on the referenced side;
otherwise, DELETE/UPDATE actions there fail to notice rows in the table
that was partition, and so are incorrectly allowed through.  With this
commit, those triggers are now created.  Conversely, when a table that
has a foreign key is attached as a partition to a table that also has
the same foreign key, those action triggers are no longer needed, so we
remove them.

Add a minimal test case verifying (part of) this.

Authors: Amit Langote, Álvaro Herrera
Discussion: https://postgr.es/m/f2b8ead5-4131-d5a8-8016-2ea0a31250af@lab.ntt.co.jp