[ticket/12831] Optn to prevent setting HTTP headers

https://tracker.phpbb.com/browse/PHPBB3-12831

PHPBB3-12831
[ticket/12809] Fix select button and dropdown list items in RTL.

PHPBB3-12809
[ticket/12809] Remove obsolete rules and selector.

PHPBB3-12809
[ticket/12809] Restore text-align for the pagination.

PHPBB3-12809
[ticket/12822] Remove "None" option from avatar select boxes

This option is just used for deleting avatars for which we already have
a checkbox.

PHPBB3-12822
[ticket/12822] Add method for deleting avatars and use confirm_box

The method will take care of the actual deletion process in the avatar manager.
Additionally, we'll be using a confirm box while deleting the avatar to prevent
users from accidentally deleting their avatars.

PHPBB3-12822
[ticket/12822] Fix test runs of avatar manager_test

PHPBB3-12822
[ticket/12951] Added editorconfig

PHPBB3-12951
[ticket/12831] reimplemented headers as array

https://tracker.phpbb.com/browse/PHPBB3-12831

PHPBB3-12831
[ticket/12831] simplified code, upd event block

https://tracker.phpbb.com/browse/PHPBB3-12831

PHPBB3-12831
[ticket/12831] fix CS

https://tracker.phpbb.com/browse/PHPBB3-12831

PHPBB3-12831
[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 #2730 from omniError/ticket/12831

[ticket/12831] Allow events in page_header functions to prevent the setting of HTTP headers
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
[ticket/12931] Fix general error on user registration

Fix "Illegal use of $_SERVER" general error on user registration
when $config['server_name'] is not set.

PHPBB3-12931
[ticket/12964] Correct use of undefined $row variable.

PHPBB3-12964
[ticket/12951] Made editorconfig file comments better

PHPBB3-12951
Merge pull request #2714 from prototech/ticket/12809

[ticket/12809] Fix select button and dropdown list items in RTL.