[ticket/11969] New language strings for first and last post links

PHPBB3-11969
[ticket/11969] Change topic link to unread post in prosilver

PHPBB3-11969
[ticket/11969] Change topic link to unread post in subsilver2

PHPBB3-11969
Merge remote-tracking branch 'cyberalien/ticket/11969' into develop

* cyberalien/ticket/11969:
  [ticket/11969] Change topic link to unread post in subsilver2
  [ticket/11969] Change topic link to unread post in prosilver
  [ticket/11969] New language strings for first and last post links