Replacements of @SUBSCRIBE@ in inc/lang/*/subscr_single.txt

The replacement of the "@SUBSCRIBE@" placeholder is not done anymore in the inc/lang/*/subscr_single.txt file, resulting in mails sent to users with this not-replaced placeholder.

However, based on the english language file, a "new" placeholder is used: "@NEWPAGE@"

Therefore I just changed the old "@SUBSCRIBE@" by the new "@NEWPAGE@" in all the subscr_single.txt lang files.