[WIP] extracted date format converting to a helper class

adding and reworking the conversion of:
- ICU to PHP
- PHP to ICU
- ICU to JUI
- PHP to JUI

TODO:
- [x] refactor date validator to use it
- [ ] ~~refactor JUI date picker to use it~~ will add this in a new PR to make it easier to review.

prepares for fixing #1551 
uses → use