luci-base: form.js: save parent map on opening nested modal map

Before opening (rendering) a nested modal map, make sure to save the parent
modal map in order to persist any structural uci changes, such as newly added
anonymous sections to prevent the nested map from operating on stale values
or ephemeral config section IDs.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>