Updated 1.6.1 release notes

Partial forward port of e85baa813f.
Optimise admin images
Merge pull request #1931 from funkybob/admin-image-opt

Fixes #21455 - Optimise admin images

Every bit is sacred
Every bit is great
If a bit is wasted
God gets quite irate
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.
Fixed #21391 -- Allow model signals to lazily reference their senders.