Fully abstract isAdmin

We're using a fork with a different definition of "isAdmin" (using
``meteor-roles`` rather than the boolean user.isAdmin).  ``lib/user.js``
provides abstracted methods for reading admin state, which makes it very
easy to change our definition -- except that the abstractions weren't
used universally.

This commit finishes the job of using the abstractions, and adds a few
new parts to also allow abstracting setting and updating admin-ness:

 - setAdmin: sets admin status directly on a user object.
 - updateAdmin: executes a mongo update to set admin status.
 - adminMongoQuery: the query parameter for admin-ness, for composing
   user queries with other fields.
 - notAdminMongoQuery: the query parameter for not-admin-ness.