AuthManagerSpecialPage: Don't use fixed submit/info class for submit button check

AuthManagerSpecialPage: Don't use fixed submit/info class for submit button check

It's possible that an authentication extension implements its own HTMLForm classes
for a submit button or info field. To omit the default submit button of HTMLForm
in these cases, too, it's necessary to check if one of the provided HTMLForm field
classes inherit from HTMLSubmitField or HTMLInfoField (assuming, that subclasses
in extensions inherit from one of them).

Bug: T137236
Change-Id: Id0ec914088b527a9c7224d545b04986068138f93