fix: Warning time error, refs #1283

* warnings.php
  * expires_period was accessed as numeric value, but is a string
* inc/datahandlers/warnings.php
  * improved handling of expiry for custom warning reasons
* inc/languages/english/datahandler_warnings.lang.php
  * added error message for invalid expires_period
Merge pull request #1285 from Nik101010/fix-1283-feature

fix: Warning time error (#1283)