Replace flow-ui- classes with mw-ui- classes

Mobile loads MediaWiki UI but not Flow UI. We should aim to get
Flow UI more in line with MediaWiki UI, upstreaming what is useful and
what is not as a separate concern.

This will also motivate us to upstream :-)

Uses core support for:
    mw-ui-quiet
    mw-ui-button
    mw-ui-progressive
    mw-ui-constructive
    mw-ui-destructive

Makes flow-ui-inline a generic class not tied to buttons
Remove flow-ui-thin

Leave concepts that do not exist in core yet in this patchset
Existing styles left in for caching reasons.

Dependency: Ia6deef500ada2cf33c96ce70909592764df39901
Change-Id: Ia98b5cc4dabaddfb7cff1720740ac0789111cb96