symmetric relations: replace bogus rql2sql translation by a hook The hook ensures X r Y => Y r X iff r is symmetric. The rql-no-hook data importer receives a small amendment but note that: * there exist no test for it * its actual semantics are undefined Hence we cannot prove this hunk breaks nothing, because we cannot prove anything. Closes #3259713.

[migration] fix handling of default value for boolean attributes We can't assert that the old value is 'True' or 'False', because False used to be stored as an empty string in pre-3.18 versions.

[entity/url publishing] fetch_rqlst should use 'is_instance_of' instead of 'is' We usually want to get any subclasses as well (think Training subclass of Workcase). Closes #3210365.

[rewriter] fix latent bug: arbitrary etype may be substituted when using is_instance_of type restriction Note: the test is not deterministic because the bug depends on the iteration order on a set.