Add a way to unlock content while saving it

[davisagli]

Currently when an item is saved in Volto, it makes 2 separate requests: one to update the content, and one to unlock it.

1. User clicks save
2. Volto sends a PATCH to the content URL and a POST to [content url]/@unlock
3. When the PATCH is done, Volto navigates back to the content view, which does a GET of the content URL

If the unlock has not yet finished at step 3, some actions might not be available yet in the GET response, since the content is still locked.

To avoid this, we can add a way to unlock the content in the same transaction where it is updated. Maybe an X-Plone-Unlock header or something.