luci-base: uci.js: fix section deletion logic

luci-base: uci.js: fix section deletion logic

 - Process deletions before additions or changes, allowing user code to
   remove and recreate a section with the same name.

 - Only record section deletions when the section to be removed actually
   existed in the original config or when it was staged for creation
   earlier. This avoids stray ubus not found exception when saving.

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