[ticket/10896] Adds email validation to email settings in ACP

Adds a new validation type to the ACP validate_config_vars function
and implements it on the board_contact and board_email settings.

PHPBB3-10896
[ticket/10896] Move EMAIL_INVALID_EMAIL to common, replace EMAIL_INVALID

PHPBB3-10896
Merge remote-tracking branch 'github-exreaction/ticket/10896' into develop-olympus

* github-exreaction/ticket/10896:
  [ticket/10896] Move EMAIL_INVALID_EMAIL to common, replace EMAIL_INVALID
  [ticket/10896] Adds email validation to email settings in ACP