[ticket/11959] Use plurals for the language strings.

PHPBB3-11959
[ticket/11959] Fix the NOTIFICATION_QUOTE_TRIMMED text.

PHPBB3-11959
[ticket/11959] Use the plurals system for the untrimmed language strings.

PHPBB3-11959
[ticket/11959] List the last user with "and" instead of a comma.

PHPBB3-11959
[ticket/11959] Add samples for the untrimmed strings.

PHPBB3-11959
[ticket/11959] Add function to create a string list.

PHPBB3-11959
[ticket/11959] Simplify how the users are trimmed.

PHPBB3-11959
[ticket/11959] Rename phpbb_gen_string_list() to phpbb_generate_string_list().

PHPBB3-11959