Ensure Setting :default values match the Setting :type; fix user-facing-value...

Ensure Setting :default values match the Setting :type; fix user-facing-value for calculated Settings. Cleanup & extra tests for email settings (#17442)

* Change of plans. get-string (etc) only return default if it matches the type

* Better docstrings

* More tests & rework the validate-default-value logic a bit.

* Fix lint error

* Add an additional test for GET /api/setting

* Consolidate exception-chain util functions

* Email error message humanization improvements

* More tests for util ex-chain

* Clean up email error handling and add lots of extra tests

* Fix docstring indentation

* Even more email tests

* Test fixes 

* Update Cypress test again

* Code cleanup