mediawiki.ui: Do not depend on divs being present inside .mw-ui-vform

This breaks forms that try to use semantic elements like <section>,
e.g. the user creation form here in core.

Bug: 63233
Change-Id: Icb4a6d1d74490808b2fd1777d7dbe812e66bd206
Merge "mediawiki.ui: Do not depend on divs being present inside .mw-ui-vform"