This closes#8744 and #9826.
It necessitated some customisation options for TwoStepButton. One is inlineText, which puts the confirmation text in the same line as the buttons. Also, there was a single-use configuration option named isInfoAction that I removed in favour of passing a set of class configuration options like this:
@classes={{hash
idleButton="is-warning"
confirmationMessage="inherit-color"
cancelButton="is-danger is-important"
confirmButton="is-warning"}}
