Useless and somewhat invalid <dt> markup on the login form

The markup that was done for the login form to follow the "vertical form" standard macro in XWIKI-10698 uses a <dt> element as the last element of the <dl>, for the submit button.

According to https://html.spec.whatwg.org/multipage/grouping-content.html#the-dt-element , the dt element needs to be followed by an element in the dl, another <dt> or a <dd>.

Also:

• nothing in https://www.xwiki.org/xwiki/bin/view/Documentation/DevGuide/FrontendResources/VerticalForms/ suggests that buttons should also be in a definition list
• it's not clear whether the name-value pair applies to the case of the button as well
• even if it would, all other inputs are <dd> elements, this is the only <dt> one, it's rather inconsistent-ish.

My proposal is to remove the dt markup from alltogether from the buttons.