mw.widgets.DateInputWidget: Allow not having a date selected

mw.widgets.DateInputWidget: Allow not having a date selected

* Allow empty value, that is, no date being selected. Display
  an appropriate label when that is the case.
* Start with no date selected, rather than today's date.
* When the field is empty and gets focus, set the date to today, on
  the assumption that this is likely what the user wants. Permit
  emptying the field, though.
* Highlight today's date on the calendar.

Bonus cleanup:

* Update the UI from #setValue, immediately when needed.
* Improve validation of incoming values.
* Correct some documentation.
* Correct some bad copy-paste in styles which caused disabled widget
  to not display correctly.

Change-Id: I7a1f7ff20eb6fc21ea59ecfe48deb9305c8e29e8