Use valid html5 input elements in forms

The HTML input elements used in phpBB use the size and maxlength settings incorrectly. They should be used as outlined here:
http://www.w3.org/TR/2014/REC-html5-20141028/forms.html#the-input-element

For example, inputs of type number should not have the size or maxlength attributes set but rather use min and max.