Previously there was no symmetry in the name of the share endpoint,
which is meant to be used to generate a publicly-accessible URL to view
a node, and the previously-called "unpulish" endpoint.

The problem is that now the "unpublish" name would clash with the dual
of publishing a node. To avoid the problem, this commit renames the old
"unpublish" endpoint to be "unshare", so that now it's clear that is the
inverse operation of "share".