Fixes #425 (closed). General improvements:

• Fixed a bug in the unpublishing of a node;
• Added the NodePublishPolicy to the NodeTree data structure, so that the frontend is aware of the type of publishing;
• BREAKING: rename /unpublish endpoint into /unshare, for better clarity of intentions;
• Add /node/:node-id/publish endpoint (by specifying the policy);

I have decided to leave outside this MR the following:

• TODO: Support publishing of notes. This should be tracked by #402 already.
• TODO: Hide published nodes from private/share view -- need more input from Alexandre.

Companion Frontend MR: purescript-gargantext!484 (merged)