Vue merge request list app backend should support assignee wildcard IDs

Context

The User filter token (and the Assignee filter token) each provide options for Any or None.

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 assignee.

Problem

The current Vue MR list GraphQL implementation/resolver does not support this use case, which means the front end cannot use either the User or Assignee filter tokens.

Request

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

The current MR list filtered search bar supports this option.