LP#1831780: various improvements to the Angular date-select widget

LP#1831780: various improvements to the Angular date-select widget

Styling
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- the widget is now narrower
- the widget now enables form validation styles; in particular,
  entry of an incorrectly-formatted date is now highlighted
- the calendar drop-down is now allowed to overflow the containing
  element when expanded, making it easier to embed the date
  selector in other controls
- the calendar button (and any material icons button that's part of
  an input group) now has the same default font size as main text,
  making the overall date-select look cleaner

API
---
- add a reset() method
- hitting enter in the text box can now triggers emitting date
  change events
- a new onCleared event is emitted if the suer hits enter on an
  empty input
- onChangeAsYmd() now pads month and day to two digits apiece,
  making the result conform to ISO 8601 and thus more easily
  plugged into queries.
- adds the following methods to retrieve the current date; these
  are meant to be used via local template references in parent
  templates:

  currentAsYmd()
  currentAsIso()
  currentAsDate()

Sponsored-by: MassLNC
Sponsored-by: Georgia Public Library Service
Sponsored-by: Indiana State Library
Sponsored-by: CW MARS
Sponsored-by: King County Library System
Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>