Adds a new centralized formatting class for datetime data, adjusting various classes to make use of the new formatters.

Applied formatter to system settings processor and made a few other corrections to previously-committed code.

Should have included this with the last commit

Include empty date display value in early return

Applies formatter to this object type

(Finding areas I'd missed updating in earlier commits)

Add/tweak method documentation; make nullable string more explicit in method signatures

Applies new modManagerDateFormatter to Context settings

Make requested changes and add a few more prop/method type declarations

Implement requested change missed in previous commit

… into 3.x-issue-14961