Vue merge request list app backend should support milestone wildcard IDs

Context

The Milestone filter token provides options for None, Any, Upcoming, or Started.

These are "special" queries that report to the GraphQL as some other key.
By convention so far, this key is called [X]WildcardId where X in this case would be wildcard.

Problem

The current Vue MR list GraphQL implementation/resolver does not support this use case, which means the front end cannot use the Milestone filter token.

Request

The back end should implement the milestoneWildcardId GraphQL implementation for the MR list resolver.

The current MR list filtered search bar supports this option.