Update the data for the current element after saving it instead of rerunning the whole query.

[sabina-talipova]

Description

Update SaveAction's endpoint function to be able to update the data for the current element instead of rerunning the whole query.
See:

silverstripe-elemental/client/src/components/ElementActions/SaveAction.js

Line 52 in 2a0ba78

// @todo optimistically update the data for the current element instead of