Allows FormBox to contain fields outside "left" group.
The use of this is most evident when FormBox is in the "bottom" group: it
allows included form to have field groups with borders (left, right,
center) or without borders (bottom), hence allowing competely extending
base form. This required rewriting JS code stitching "left" & "right"
groups together so it honours nesting.
For backward compatibility, hide "left" groups rendered inside any bordered
group - but only when there is no field in "right" group.
Also, clean up duplicated code in embedded_form_render and use proper
naming (templates used by forms should always end with "_view").