[BREAKING CHANGE] Return promise instead of taking callback argument

In the OO.ui.OptionWidget flash method, we now return a promise.

Technically this is a breaking change, but there are no known callers
outside of OOUI.

Change-Id: I2dcddb9199c225ba2ebc89a1d2d1f144cfb38122
Merge "[BREAKING CHANGE] Return promise instead of taking callback argument"