[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() & fix incorrect var name.

PHPBB3-11959
[ticket/11959] Remove use of plurals and make it possible to use Oxford comma.

PHPBB3-11959
[ticket/11959] Add unit tests.

PHPBB3-11959
[ticket/11959] Move phpbb_generate_string_list() to functions_content.php.

PHPBB3-11959
Merge remote-tracking branch 'prototech/ticket/11959' into develop-ascraeus

* prototech/ticket/11959:
  [ticket/11959] Move phpbb_generate_string_list() to functions_content.php.
  [ticket/11959] Add unit tests.
  [ticket/11959] Remove use of plurals and make it possible to use Oxford comma.
  [ticket/11959] Rename phpbb_gen_string_list() & fix incorrect var name.
  [ticket/11959] Simplify how the users are trimmed.
  [ticket/11959] Add function to create a string list.
  [ticket/11959] Add samples for the untrimmed strings.
  [ticket/11959] List the last user with "and" instead of a comma.
  [ticket/11959] Use the plurals system for the untrimmed language strings.
  [ticket/11959] Fix the NOTIFICATION_QUOTE_TRIMMED text.
  [ticket/11959] Use plurals for the language strings.