Regression: align-self: stretch change is affecting horizontal flex

[web-padawan]

The change landed in vaadin/vaadin-form-layout#107 was targeting the form-layout placed in the element with flex-direction: column, but it was actually breaking change from the point of view of the users who try to center form-layout in the container setting flex-direction: row

This should have been released as 3.0.0 but we didn't do that. The proper way of handling this would be reverting change, releasing 2.1.3 and then putting that change back as 3.0.0

At the same time, we also have to document what form-layout is NOT supposed to be direct child of a flex layout (as it has to calculate its own width, and flex affects that)