Add optimistic cache updates to saving blocks

[ScopeyNZ]

In elemental ^4.

Now there's a button to save in-line edited blocks individually with #296. This is awesome but it still reloads the list of blocks on save. Preferably we can update Apollo with optimistic cache updates and avoid the reload.

[robbieaverill]

My initial attempt with narrative: https://github.com/dnadesign/silverstripe-elemental/pull/392#issuecomment-422897050