Add "storm" option for the span property

[webbati]

Now is not possible to set "storm" as span value and the builder plugin delete the value saving after you set it manually.

We also apply the "cssClass" property to the widget of the builder if "storm" value is selected for the span property, default class "col-sx-12 col-md-6" will be applied if nothing is set

[LukeTowers]

We should probably call it manual instead, storm isn't really a special value that's treated any differently if I recall, it's just the use of any non-supported values that triggers the behaviours associated with setting span: storm.

[bennothommo]

@LukeTowers you're right, also, it looks like auto doesn't do anything either - at the very least, I haven't found any code that directly targets it.

[josch0]

I stumbled across this.
To keep the builder plugin consistent with the docs, there really should be the “storm” option available.

This is the only/preferred way to use the bootstrap grids as a cssClass, right?

@LukeTowers
Is there anything against merging this?

[LukeTowers]

@josch0 just the same as my previous explanation. There really is nothing about the storm value that makes it special, it's just that it isn't any of the other values. https://github.com/wintercms/winter/blob/f1981631da4b8ebd7173eee41b2ea995b7d4700d/modules/backend/classes/FormField.php#L89