Use GraphQL `autocompleteUsers` in assignees widget

Currently, we compose a list of options in assignees widget manually the combining current user and search results. Instead, we should use a new autocomplete query:

query AutocompleteUsers ($project: ID!) {
  project(fullPath: $project) {
    autocompleteUsers {
      id
      username
      name
    }	
  }
}

Availability and Testing

Regression testing, please make sure e2e:package-and-test job is passing.

Ensure updated test coverage in unit/integration/feature tests.