Fixed #21404 -- Added session.set_expiry() note to 1.6 release notes.

Thanks pwr for the suggestion.
Fixed #21240 -- Added 1.5 release note for OneToOneField/select_related change.

Thanks marcin at sokrates.pl.
Added release note for TypedChoiceField coerce limitation

Thanks Elec for the report and Simon Charette for the review.
Refs #21397.
Fixed #21397 -- Re-added flexibility to TypedChoiceField coercion

Thanks Elec for the report and Simon Charette for the review.
Merged the signals and signals_regress test packages.

This patch also made the tests less likely to pollute the global state
in case of failure.
Added more internal links in the management command documentation.