[ticket/12932] Add method for getting the datetime class and allow overriding

An extension can now override the datetime class by overwriting the
datetime.class parameter in their own yml file.

PHPBB3-12932
[ticket/12932] Fix tests and calls to create_datetime

PHPBB3-12932
[ticket/12932] Remove set_datetime_class method from user class

PHPBB3-12932
Merge pull request #2862 from marc1706/ticket/12932

[ticket/12932] Add method for allowing override of \phpbb\datetime class

* marc1706/ticket/12932:
  [ticket/12932] Remove set_datetime_class method from user class
  [ticket/12932] Fix tests and calls to create_datetime
  [ticket/12932] Add method for getting the datetime class and allow overriding