New js component DateTimeInput to fix the invitation form

The invitation form now uses multiple inputs (<select>s) to select a time
and a datetimepicker to select a date (the same way is done for events in
mweb_events).
These inputs are grouped together in a new js class DateTimeInput.
Not finished yet, but the js part of the form is almost finished.
Finish separate date/time inputs in the webconf invitation form

Last overall changes in the controller, js and view to use separate inputs
for dates and times in the webconf invitation form.

Complements 1eb47cf09