Allow projects to inherit variables, including secrets, from a different source.

Some Evergreen projects share several variables among them.  At present, Evergreen offers two REST routes to manage variables which allows copying variables to an existing project and rotating variables . 

However, there's currently no UI-driven mechanism to share variables across projects.  Lack of cross-project variables management creates administrative burden for project owners as they need to copy/paste variables across projects and update variables across several projects manually. Evergreen could potentially provide an interface to allow cross-project variable administration easier.