[user role assignment] offer a "Verify user ID" when no results found from cache

[ArtieReus]

Use case
Right now when assigning roles to a user the user itself have to be first searched by name or ID (see picture below), selected and afterwards assign the different roles to that user.

Problem
The search functionality uses a local cache which it is filled when users have been logged into elektra a least once. Since technical users are not allowed to log in elektra the search will always return no results. The user will be still able to assign roles to the user entered in the search box, even if is not found, but since no results are returned it feels like wired.

Proposal
When searching for a user and no results have been found an extra button will be displayed in the dropdown (see picture below). This button should verify per API that the given user ID exists and allow to assign roles. An error should be displayed (ex: No user found with ID "arturo") if the user tries to verify a user name instead of an id. The API call should be just against user IDs.

This should fix #1198