[v24] Translation support improvements

* Dynamic HTML titles (index / thread pages), the date format
('dd/mm/yyyy') and other strings that were previously
not translatable (since v19) are now translatable. This causes a number
of changes in theme configuration:
- These strings are now stored in `$LANG['']` (the default language) in
'theme.php' instead of, as before,
as constants in 'theme.config.default.php', therefore it is advised that
you make a new 'theme.config.php'
file by making a copy of the updated 'theme.config.default.php' file
- If you've modified the 'theme.php' file at all, you should update to
the new version
- If you've written any translations, you will need to update those with
the new set of strings at the top
of 'lang.example.php'
* Fixed duplicate ID `nnf_post`, with thanks to Bruno Héridet
NOTE: `<article id="nnf_post" class="...` was changed to `<article
class="nnf_post ...` in "append.html",
"delete.html" and "thread.html". No changes required in the CSS.
* Fixed critical typo in "lang.example.php" with thanks to "gardener"
* Pages were not working on sub-forums, with thanks to Steve Bir
* Removed extra space that was being added around code blocks