Schedule: add AJAX throbber to Date filter

Problem:
It can confuse user: filter was changed, the rest of form is inactive but no throbber appeared.
For now Schedule form's Date filter just clicks on Submit button using JS: https://github.com/ymcatwincities/openy/blob/8.x-1.x/modules/custom/open...

Solution:
Form field with date picker should have its own AJAX callback on change event that will show throbber in needed place.

Please review PR that fixes it https://github.com/ymcatwincities/openy/pull/859

Also please have a look to video demonstration how it improves UX (in attachment)