[FIX] web: Separate datepicker inputs

This commit splits the input element of the DatePicker and
DateTimePicker components into 2 separate inputs: the first one which is
the one used for parsing and which will actually be displayed in the UI,
and the second (type="hidden") that will be used by TempusDominus.

This has been done in order to fix major communication issues between
Moment (used by TD) and Luxon (used in the rest of Odoo), which up until
now have been resolved with the help of an intermediate "static format",
which itself caused a whole bunch of other problems.

This solution should hopefully be more seemless and less error-prone.

Fixes the following reports:
* OPW-3278019
* OPW-3295290
* OPW-3316325 (partially)

Part-of: odoo/odoo#123245
Co-authored-by: Jorge Pinna Puissant <jpp@odoo.com>
Co-authored-by: Julien Mougenot <jum@odoo.com>