HTMLForm: Use <button> and allow differing label and value

HTMLForm: Use <button> and allow differing label and value

Submit buttons, whether via HTMLForm::addButton() or HTMLSubmitField,
are difficult to use effectively when the submitted value has to match
the internationalized display label. HTML5 (and HTML4 for that matter)
already has <button> that does what we need, we just need to actually
use it.

But for IE6 and IE7, we can't use <button> because the browser doesn't
handle them correctly. Sniff those browsers to avoid their bugginess
(which the OOUI version should have been doing already for IE6, but
wasn't).

Bug: T121584
Change-Id: I7e15331efb162275c4116bcae61f19d6b884cbe3