luci-base: add full page overlay during apply

After applying uci configuration, a full map reload is required in many
cases as the anonymous section identifiers might have been rehashed, causing
the rendered map to go out of sync.

To avoid that, add both a full page overlay preventing further page
interaction and let the apply widget forcibly reload the current view once
the operation is complete.

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