[2.6] bootstrap form themes and form_label with checkbox

[weaverryan]

We should note that form_label should not be used on checkbox fields - it renders nothing - symfony/symfony#13501

[weaverryan]

Bah, nevermind, this is already included - I was looking on the wrong branch :). I'll update the issue to just be about form_label.

[stof]

You should not that this depends on the theme (the default theme does not render the label inside the widget. Only the bootstrap theme does)