[ticket/10073] Use get_username_string and correctly sprintf for languages

PHPBB3-10073
[ticket/10073] Fix title for "Email topic"

PHPBB3-10073
[ticket/10073] Fix missing language string

PHPBB3-10073
[ticket/10073] Fix invalid type hint

PHPBB3-10073
[ticket/10073] Add configs for contact info text

PHPBB3-10073
[ticket/10073] Display contact info

PHPBB3-10073