The concat_date function would return somewhat weird results if any of
the year, month or day parameters were zero. For example, in the generated
project templates the date would be shown as "0-12-00" if the user did
not select a date. The code in Administor_projects.php avoided this by
explicitly checking if any of these components were zero. This check
has now been moved to the concat_date function itself so that the logic
does not need to be repeated in multiple spots. (This function is currently
only used in two spots.)

The updated concat_date function now also makes more use of the standard
date and mktime functions instead of more simple string manipulation. This
increases the chance of getting a valid date as a result with more
consistent formatting. For example, 2021-02-29 (not a valid date) will now
get converted to 2021-03-01. See mktime documentation for more info.

The concat_date function now also accepts an optional parameter (defaults
to 'Y-m-d') to allow other date formats to be used.

Previously the poetry templates would always show "Sunday, DATE" (or a
translated equivalent) as a placeholder that then needed to be manually
updated.

Note that the generated date text in the template output is in English
even if another language is selected. It could be a lot of work to generate
these strings properly in all the languages and currently the poetry
templates are only being used for English. Even if these are used for
non English the results are no worse than before as they already needed
to be manually updated to replace the fixed placeholder.

This fixes LibriVox#124.