when adding collabs to projects, show "xx mutual collaborators"

[williamstein]

How cocalc works now:

How facebook works:

Add additional info to the returned object showing the "number of mutual collabs", which is by definition the cardinality of the intersection of all collabs you have on projects with all collabs they have on projects. I do not know how hard that "graph query" is to do, but it's an intersting challenge...