Fix Button label in OOUI HTML Form and enable flags

An OOUI Button now adds the value as a label of the button, too. Enable
the HTMLForm descriptor to pass flags to the OOUI Button widget, which
defaults to nothing in HTMLButtonField and to 'primary' and 'constructive'
in HTMLSubmitField.

Change-Id: Ibf561453604a830862d4f64819c8418a9458cb12