Align to WikimediaUI themed widgets

Align to WikimediaUI themed widgets

Aligning to WikimediaUI themed widgets. As of now, Calendar- &
DateTimeInputWidget have been user-interface wise in limbo between
WikimediaUI style and non-themed style. Let's align it to former
and ensure theme-dependency in a future step.
Also
- massively cleaning up LESS,
- removing duplicated, already inherited properties and
- fixing little annoyances like `line-height` in fields being wrong or
- `:focus` states being disconnected from the rest.

Bug: T189352
Change-Id: Ia39254aec3535aed9e3d3e01b3fc01c25cd72c2e