Improve date_strtotime()

Following the logic introduced in dbafafa9d98c9cd1dfe3642b5dcbb0a5ef925f6e,
the function now relies on DateTimeImmutable instead of strtotime() to
convert the string to a Unix timestamp.

It starts the conversion attempt by using $g_normal_date_format, and if
that fails falls back to PHP Supported Date and Time Formats.