mediawiki.ui: Rename .mw-ui-vform-div → .mw-ui-vform-field

mediawiki.ui: Rename .mw-ui-vform-div → .mw-ui-vform-field

Require use of class instead of applying styling to all <div>s

This resolves a few FIXMEs and dramatically lowers the awkwardness
level of using mediawiki.ui.

'.mw-ui-vform-field' is a more descriptive name than '.mw-ui-vform-div'
and corresponds to the HTMLFormField PHP class in core which generates
<div>s with this CSS class.

We previously styled '.mw-ui-vform > div' the same way we styled
'.mw-ui-vform .mw-ui-vform-div', which was an annoying piece of magic
causing difficult to debug problems when one needed a different HTML
structure (like bug 63233). Explicitly using '.mw-ui-vform-field'
where applicable is a lot saner.

Change-Id: I6f0b8842f5fdf70b97decb165086d1a83428b259