[ticket/13312] Add event core.acp_email_send_before

PHPBB3-13312
[ticket/13312] Use empty for usernames array

PHPBB3-13312
Merge pull request #3191 from Elsensee/ticket/13312

[ticket/13312] Add event core.acp_email_send_before
Merge branch 'develop-ascraeus' into develop