CAS option when editing documents in Console UI - needs ns_server support in document CRUD API

Summary: When editing documents in the Console, there should be an option to 'Save and check CAS'

Repro:

1. Open Console in two browser tabs
2. Open the same document in both tabs.
3. Make a change to the first document and hit 'save'
4. Make a different change to the second document and hit 'save'

Actual:

The second change overwrites the first change. CAS is ignored.

Expected:

The second change would have a new button (or something) to save with CAS check. Clicking this instead of the normal 'Save' would display an error.