Exclude current project from "add user" typeahead

The user typeahead now takes an optional argument, the project ID to
exclude (or a list of project IDs to exclude). If not specified, all
users will be searched, but if a project ID (or a list of IDs) is given,
then only users who do not belong to the project (or to any of the given
projects, if plural) will be shown in the typeahead.

This fixes bug #100.