Use date_strtotime() instead of strtotime()

Replace native strtotime() calls by revised date_strtotime() where
relevant, i.e. when date string could be user-provided.

This prevents occurrence of the error described in issue #31866.

It also allows users to submit dates in $g_normal_date_format in the
following situations:

• REST API: Create a project version (timestamp), Add Issue (due_date)
• SOAP API: Add/Update Issue (due_date) and Project Version Update
  (date_order)
• Bug Update, Bug API (due_date)
  NOTE: change to bug_update.php is not strictly not necessary as the
  script is normally only called internally, not directly by users.

TODO: the REST API's documentation should be updated