cubicweb #5457548 button css class isn't overriden as it should be by bootstrap [validation pending]

bootstrap views/formwidgets.py overrides default form buttons css classes.

In the case of a button created for a reledit auto form, which is created earlier, the css class is written in Button.attrs dict (see cubicweb/web/formwidgets.py) and read from there to render it. So when bootstrap, loaded afterhand, overrides the css class, this operation has no effect on the rendering.